[physics515]

Completes the PAC-5a/PAC-6 configuration registry (1.0-plan §3 PAC-5a/PAC-6 row + §6 W8 open lane + §8 wording per decision 21). No ROADMAP line exists for this slice; issue #439.

What changed

• TEST_DATA_DIR VarSpec removed from the registry in weft-server/src/settings.rs (registry 104 -> 103; size assertion and Test-tier allowlist updated).
• weftdb::data_dir drops the TEST_DATA_DIR leg; test suites are handed their fixture directory through the Stable WEFT_DATA_DIR variable instead. Every suite/bench that set TEST_DATA_DIR now sets WEFT_DATA_DIR (weftdb, weft-server, weft-orchestration tests and benches).
• Docs: config schema, reference, CHANGELOG, and a DECISIONS.md entry recording the replacement-channel choice, flagged FOR MERGE REVIEW.

Verification (full bar on this exact tree)

• cargo +nightly fmt --all -- --check — pass
• RUSTFLAGS=-D warnings cargo check --workspace --all-targets --locked — pass
• clippy workspace --all-targets with -D correctness/suspicious/perf — pass
• rustdoc -D warnings — pass
• tests: weftdb + weft-server + weft-orchestration — all pass, 0 failed
• claims lint (docs/reference.md) — pass

Salvage note: implemented by the W8 loop, whose run timed out; the parent salvaged the worktree diff, re-verified, rebased onto current main (DECISIONS.md union) and re-ran the full bar green before publishing. Per the wave batch pattern this PR stops for the wave-end batch merge.