fuzz: wave 1 — infra + bast_compile/data_access targets, seeds, corpus-replay gate

- fuzz/ workspace (nightly-pinned subtree, own [workspace]), copied
  from the alkhttp/alkcall pattern: thin fuzz_target wrappers,
  stable-toolchain shared crate holding the invariant logic, detached
  runner, seed generator, json.dict, README
- bast_compile: AlkTypeEngine::compile both modes over attacker BAST
  JSON; meta-schema gate ordering, always-Result, fixed-size leaf
  metadata partition, json_schema lane
- data_access: the hand-rolled decode core over raw bytes at
  attacker-chosen offsets; bool strictness, UTF-8 discipline, bounds
  partitions, indirect {offset,length} pair contract, write-side
  no-touch-on-failure + write/read round trips
- 136 committed seeds (38 + 98) via fuzz/gen_fuzz_seeds.py
- root Cargo.toml: explicit [workspace] exclude=[fuzz]; publish
  exclude gains fuzz/
- AGENTS.md verification checklist gains the corpus-replay gate
- .gitignore: fuzz artifacts + grown-corpus pattern

Verification: cargo test 569 pass; clippy -D warnings clean; corpus
replay 4/4 green (136 seeds); cargo fuzz build clean (nightly
confined to fuzz/)

AGENTS.md

@@ -165,8 +165,18 @@ cargo clippy --all-targets -- -D warnings
 cargo doc --no-deps                                   # if docs changed
 cargo build --target wasm32-unknown-unknown --release # if layout/wasm-relevant code changed
 cargo publish --dry-run --allow-dirty                 # before a release
+cargo test --manifest-path fuzz/shared/Cargo.toml     # fuzz corpus replay (the fuzz gate)
 ```
 
+The corpus replay is the standing fuzz gate (the alkcall
+`docs/research/fuzzing.md` §7.9 posture): it replays every committed
+seed through the same invariant functions the fuzz targets run, on
+stable, without nightly. Campaigns (nightly, cargo-fuzz) run manually
+via `fuzz/run-detached.sh` — never as a foreground child of an agent
+session — before releases, after touching `src/data_access.rs`,
+`src/schema.rs`, the compile walks, or the sequential reader. See
+`docs/plans/fuzzing.md` and `fuzz/README.md`.
+
 ## Architecture Context
 
 - `docs/architecture/` — the authoritative spec. Read it before