Merge pull request #1616 from HighDrexler/fix/large-required-imports-v2

Fix/large required imports v2

docs/rfcs/0008-streamed-mod-imports-and-install-cache.md

@@ -0,0 +1,141 @@
+# RFC 0008 — Streamed mod imports and installation-scoped generated cache
+
+## Motivation
+
+`required_imports`/`optional_imports` can now describe files up to 2 GiB, but
+the existing launcher and public mod API still assume imported bytes are small:
+
+* the Windows desktop picker stages a selected required import through a fixed
+  `%TEMP%/pokeport_required_import.bin` path before validation;
+* the fallback import path materializes the selected file as one Lua string;
+* after validation a mod can only use `mod:read("baseroms/...")`, which also
+  materializes the whole file;
+* `mod.storage` is intentionally scoped to one Pokémon playthrough, so it is
+  not an appropriate home for a one-time generated asset cache shared by every
+  save using the same installed mod.
+
+This makes optical-disc-sized user sources impractical even though the manifest
+schema already accepts them. A failed temporary staging copy can also turn a
+valid large source into a smaller temporary file and produce a misleading
+"wrong file size" rejection.
+
+A mod should be able to consume its own already-validated source incrementally
+and compile derived runtime data once, without receiving a host path or general
+filesystem access.
+
+## Decision being extended
+
+This extends the same legal/sandbox direction as **D11 asset transforms**
+(`src/mods/AssetTransform.lua`): mods distribute recipes and derive bytes from
+user-owned sources rather than shipping ROM-derived data. It also follows the
+**D14 parity-gate** contract referenced by `tests/harness.lua` and
+`tests/engine/gate_meta_coverage.lua` (the `21-testing-and-ci` plan): additive
+extension points ship public-API coverage, no-mod parity coverage, and docs in
+the same change.
+
+The historical D11 plan document is referenced by source comments but is not
+present in the current repository tree; this RFC is the checked-in design
+record for the new surface.
+
+## Exact API delta
+
+No manifest field changes. Existing `required_imports` and `optional_imports`
+remain the declaration/validation authority.
+
+Two additive facades are added to the `mod` object.
+
+### `mod.imports`
+
+```lua
+local info, err = mod.imports:info("source_id")
+local bytes, err = mod.imports:read("source_id", offset, length)
+```
+
+* `source_id` must name an import declared by the calling mod.
+* the import is rechecked through `RequiredImports.validateStored` before it is
+  exposed, so missing, replaced, or invalid optional imports are not readable;
+* `offset` and `length` are zero-based byte coordinates;
+* one read is capped at 8 MiB;
+* no host path or file handle is returned;
+* production reads seek into the engine-owned stored copy instead of reading
+  the whole source.
+
+`info()` returns declaration metadata plus stored size. It does not expose a
+host path.
+
+### `mod.cache`
+
+```lua
+mod.cache:write("extract/v1/model.bin", bytes)
+local bytes = mod.cache:read("extract/v1/model.bin")
+local info = mod.cache:info("extract/v1/model.bin")
+mod.cache:delete("extract/v1/model.bin")
+```
+
+The cache is rooted at `mod_cache/<mod-id>/`, follows the engine persistence
+backend, and is independent of game version, launcher slot, and playthrough.
+Paths are checked with `SafePath`; `..`, absolute paths, drive paths, and other
+escapes remain unavailable. A single cache write is capped at 64 MiB so large
+generated datasets are naturally split into independently replaceable files.
+
+The engine does not interpret cache bytes. Mods own generated-format versioning,
+fingerprints, transactional completion markers, and rebuild policy.
+
+## Launcher/import transport delta
+
+For large raw required imports:
+
+1. desktop pickers return the original selected path instead of staging it
+   through a fixed temporary file;
+2. the engine opens that source itself;
+3. bytes are copied directly to the existing engine-owned
+   `mods/<id>/baseroms/<file>` destination in 4 MiB chunks;
+4. MD5 is updated incrementally during the copy;
+5. the normal size/MD5 validation receipt is written only after the complete
+   destination passes validation;
+6. partial destinations are removed on short reads, write failure, size
+   mismatch, or digest mismatch.
+
+N64 imports stay on the existing canonicalization path because byte-order and
+copier-header normalization require transformation rather than a raw copy.
+
+If a validation receipt for an already-stored large raw import is missing, the
+engine rebuilds it with streaming MD5 rather than a whole-file read.
+
+## Backward compatibility / migration
+
+**Existing mods do nothing.** This is additive.
+
+* manifest v1/v2 fields are unchanged;
+* `mod:read`, `mod.storage`, registries, events, hooks, and legacy compatibility
+  retain their existing behavior;
+* small required imports retain the existing in-memory validation path;
+* N64 imports retain canonicalization and existing accepted byte orders;
+* a mod that never touches `mod.imports` or `mod.cache` creates no new cache
+  files and observes no new behavior.
+
+The mod API integer is not bumped because no existing member changes meaning or
+shape.
+
+## Security and legal posture
+
+The launcher remains the authority that validates user-supplied bytes. The new
+facade narrows access rather than widening it: a mod can read only ids declared
+in its own manifest, only after validation, and only in bounded ranges. It does
+not receive host paths, `io`, or a raw filesystem handle.
+
+`mod.cache` is writable only beneath the calling mod's generated-cache root.
+Nothing in this RFC permits packaged ROM-derived bytes; `modkit lint/pack`
+continue to enforce the existing legal posture.
+
+## Parity guarantee
+
+The change ships with:
+
+* a no-mod/API-v1 parity test proving an empty load and an existing v1-style
+  `mod:read` load do not create cache data or change the old surface;
+* a public mod-API test that reaches `mod.imports` and `mod.cache` through a
+  real `Loader` load, including bounded reads, undeclared/missing imports,
+  cache isolation, and traversal rejection;
+* incremental MD5 vectors and a large-import streaming regression test;
+* the existing engine suite, required-import suite, and mod lint gates.