fix: no-default sim phantom-exclude + gate template crates under -D warnings

[heyoub]

Two mutation/CI-lane hygiene fixes, each a separate commit.

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1. fix(mutants) — stop the no-default surface grading the cfg-gated sim tree (phantom survivors)

store/sim/** is entirely #[cfg(feature = "dangerous-test-hooks")]-gated
(store/mod.rs:55), so under --no-default-features the whole tree is compiled
out. cargo-mutants (27.1.0) does not evaluate cfg — cargo mutants --list
reports the same 3976 mutants for both surfaces, 359 of them under store/sim/.
On the no-default surface every one of those 359 lands in dead code: build + tests
pass unchanged, so the mutant is scored a phantom "missed" that can never be
caught there — ~9% of the population dragging the score down. (The "15 sim missed"
seen during the #121 cure was just the slice one shard surfaced.)

Fix: a surface-specific --exclude crates/core/src/store/sim/**/*.rs for the
no-default surface only. All-features keeps mutating sim (its dangerous-test-hooks
tests do the catching), so zero real coverage lost — only phantoms stripped.

surface	total mutants	store/sim/
no-default, before	3976	359 phantom
no-default, after	3617 (−359)	0
all-features (unchanged)	3976	359 (caught)

Also corrected a stale plan.rs comment that credited the ci profile's
terminate-after; the runner pins the mutants profile whose fail-fast = true
is the load-bearing lever.

Not a gate weakening (checked, not assumed): not a --exclude-re equivalence
claim → mutation-exclusion-registry untouched; a plain file-glob, not a raw-regex
allowlist element → ran meta-gate-check on this exact diff: "ok (no unapproved
weakening detected)"; touches no *_MUTANT_FILES include glob → glob-coverage
untouched.

2. feat(templates) — gate every template crate under -D warnings

Each crate under templates/* is its own cargo workspace, not a root-workspace
member, so the root cargo clippy --workspace -- -D warnings gate never reaches it.
The per-PR template lane (cargo xtask templates) compiled + tested each template
under plain cargo test, which does not deny warnings — exactly how a dropped
#[must_use] AppendReceipt warning slipped into two templates (fixed earlier in
e5b89da9) with nothing automated to catch it.

Fix: run cargo clippy --all-features --all-targets -- -D warnings per template
inside the existing loop (before its test, sharing the smoke --target-dir so core
is checked once). Every template is now held to the same -D warnings bar as the
workspace.

• green: cargo xtask templates runs all 10 templates through clippy -D warnings
  • test; all pass.
• red (gate bites): injecting an unused binding into a template makes clippy fail
  closed (error: unused variable → non-zero exit → lane fails); reverted.

This is the narrow per-PR lint slice of P1-B (#89); the rest of P1-B stays tracked there.

Verification (both)

cargo test -p xtask (incl. new no_default_surface_excludes_the_cfg_gated_sim_tree
pin) · cargo xtask templates green + bite-proven · structural-check: ok ·
meta-gate-check: ok · cargo fmt --check: clean · cargo clippy -p xtask -- -D warnings: clean.

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