feat(mod-api): add composable map occupancy seam

docs/rfcs/0013-map-occupancy-and-active-block.md

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+# RFC 0013: Conditional map occupancy and active-block reads
+
+## Status
+
+Proposed.
+
+## Motivation
+
+The Gen 1 Vermilion Dock script correctly ejects a player who enters after the
+S.S. Anne has departed. A content mod can add a city-side route back to that
+empty harbor, but it cannot preserve the dock visit: replacing the complete
+dock handler would discard vanilla behavior and peer handlers, while an added
+handler cannot cancel the base handler's ejection.
+
+A mod that changes one active map block also needs to prove that it is looking
+at the expected Red, Blue, or Yellow layout before it acts. `mapOverview()` is
+intentionally presentation-oriented and does not expose block identity.
+Requiring internal `Map` state or generated ROM data would cross the public mod
+boundary and make a wrong-version edit difficult to fail closed.
+
+## Decision and plan extended
+
+This extends Route B in `CONTRIBUTING-mods.md`: new behavior is additive,
+ordinary hook composition remains the authority, an empty hook chain is a
+provable no-op, and mods receive copied or scalar data rather than mutable
+engine state. It supports the approved Mew-under-the-truck implementation plan
+without adding any Mew-specific rule, asset, flag, or content to the engine.
+
+## Exact API delta
+
+### `map.occupancy_allowed`
+
+The post-departure `VERMILION_DOCK` script calls this hook after it replaces the
+ship blocks with water and immediately before it would display the departure
+message and warp the player to Vermilion City.
+
+A wrapper has this shape:
+
+```lua
+function(next, game, context) -> boolean
+```
+
+The context is a new table with these fields:
+
+| Field | Meaning |
+|---|---|
+| `mapId` | `"VERMILION_DOCK"` at this call site |
+| `reason` | Stable reason key `"ss_anne_departed"` |
+| `gameVersion` | Active save version (`red`, `blue`, or `yellow`) when present |
+| `x`, `y` | Current player cell coordinates when present |
+
+Vanilla returns `false`. The player remains only when the final chain result is
+exactly `true`. Absent, throwing, or malformed wrappers therefore preserve
+ejection. A wrapper composes by calling `next(game, context)` and returning
+true when either downstream or its own narrow rule permits occupancy. The hook
+does not replace `MapScripts` registration, merging, or dispatch, and does not
+change the departure flag or reconstruct the ship.
+
+The call is guarded by `Runtime.wantsHook`, so an empty chain allocates no
+context and follows the prior branch exactly.
+
+### `WorldAPI:activeBlockAt`
+
+Gen 1's public `mod.world` facade adds:
+
+```lua
+activeBlockAt(mapId, blockX, blockY) -> blockId
+                                      | nil, reason
+```
+
+`mapId` must equal the active map ID. Coordinates are finite, integral,
+zero-based block coordinates. A successful result is a numeric scalar copied
+from the active runtime map. The method never returns the map's mutable block
+array and never writes game or save state.
+
+Failure reasons are stable:
+
+| Condition | Reason |
+|---|---|
+| No active overworld map | `no overworld` |
+| `mapId` differs from the active map | `map is not active` |
+| Coordinate has the wrong type, is non-finite, or is fractional | `invalid block coordinates` |
+| Coordinate is negative or outside the active map | `block coordinates out of bounds` |
+| Active block data is absent or malformed | `block unavailable` |
+
+Requiring the expected map ID and rejecting all ambiguous input lets a mod
+compare every cell in its version-specific signature before it calls an
+existing mutation API. Red, Blue, and Yellow each use their own loaded map
+data. Gold does not gain this method in this RFC.
+
+## Migration and compatibility
+
+Existing mods change nothing. No hook, event, registry, manifest field, save
+field, map handler, or WorldAPI method is removed or renamed. With no hook
+subscriber the departed-dock behavior is unchanged. Existing callers cannot
+invoke the new WorldAPI method accidentally.
+
+The occupancy answer is not persisted by the engine. Disabling or uninstalling
+a mod removes its wrapper through normal owner cleanup, so a later dock entry
+uses vanilla ejection. The engine writes no new save state and neither API
+returns or serializes ROM or save data.
+
+## Verification requirements
+
+The parity gate must prove:
+
+- vanilla departed-dock message and warp remain with no subscriber;
+- one permission wrapper can allow occupancy without removing the ship-erasure
+  work or any map handler;
+- multiple cooperative wrappers preserve downstream permission;
+- absent, throwing, nil, false, and malformed hook answers fail closed;
+- Red, Blue, and Yellow contexts keep their version identity separate;
+- disabling or removing the owner restores vanilla behavior without a save
+  migration;
+- `activeBlockAt` accepts only the active map and valid in-range integral
+  coordinates, returns a scalar, and never mutates map or save state; and
+- the test fixtures and resulting changes contain no ROM or save payload.
+
+The hook must be driven through a real public `hooks:wrap` chain. The block API
+must be exercised through a public `WorldAPI` instance. Tests must not replace
+the production map handler with a test-only implementation.
+
+## Docs with the change
+
+`docs/modding.md` documents both contracts, their failure behavior, the
+cooperative wrapper pattern, and the Gen 1-only block method. No registry or
+schema changes occur, so generated registry documentation is unchanged.
+
+## Deprecation etiquette
+
+Nothing is removed, superseded, or deprecated.