effects: an in-memory file store for deterministic simulation (specs/effects/sim_fs.t27) #9012

Part of epic #9008 (effects at the edge); deterministic simulation primitives for #9002. Write it in t27 only. DRY rule (AGENTS.md, see #8963): first search for an existing implementation with t27c dupes --name <fn> and grep -rn over specs/. If one exists, use it and extend it instead of writing a copy. No hand-written code in other languages (see #8236).

Requirements

• FR-001: specs/effects/sim_fs.t27 defines a fixed-capacity in-memory store that maps a path (bytes) to contents (bytes), with these operations:
  • write(store, path, data) -> bool: creates or overwrites, and returns false when the store is full;
  • read(store, path) -> ?[]const u8: returns null for a missing path;
  • remove(store, path) -> bool;
  • count(store) -> u32.
• FR-002: A test named write_then_read_roundtrip writes two files, reads them back byte for byte, overwrites one, and reads the new contents.
• FR-003: A test named missing_and_removed_read_null asserts that read returns null for a path never written and for a removed path, and that count follows the operations.
• FR-004: Do NOT edit docs/reports/t27b_expectations.json or .trinity/seals/: they are shared hot files, and listing them made the Queen skip this issue as a file conflict. The steward adds the ledger row and re-saves the seal when the PR merges.

Success Criteria

• t27c spec-status specs/effects/sim_fs.t27 prints IMPLEMENTED
• grep -c 'test "write_then_read_roundtrip"' specs/effects/sim_fs.t27 prints 1
• grep -c 'test "missing_and_removed_read_null"' specs/effects/sim_fs.t27 prints 1

Boundary

• specs/effects/sim_fs.t27

User Scenarios

• Given an empty in-memory file store, when a file is written and then read, then the read returns the exact bytes written.
• Given a file store with two files, when one file is overwritten, then reading it returns the new contents.
• Given a file store, when reading a path never written or a removed path, then read returns null and count reflects the current number of files.