Q3 — Release Hygiene for v0.1.0

[idky137]

Q3 — Release Hygiene for v0.1.0

Intent

Put the minimal-but-real CI gates and release process in place so
limen-core v0.1.0 can be tagged cleanly and does not become a source of
breakage for downstream crates.

Decisions Made

• no_std build gate is mandatory in CI — not optional.
• CI is kept minimal but real: no speculative gates, but nothing skipped
  that would catch a regression.
• MSRV is decided and pinned before tagging; it is part of the published
  release notes.
• Minimum viable MSRV lower bound: Rust ≥ 1.65. Const-generic arrays
  [T; N] (stable since 1.51); #[non_exhaustive] on structs (stable since
  1.40); const-generic ergonomics used in NodePolicy<const IN> require at
  least 1.65 for practical usage. Final MSRV pin decided in this issue.

Depends On

• C0 — feature matrix (CI must exercise the same matrix C0 defined)
• Q1 — tests must pass before CI gates are meaningful
• Q2 — docs must build cleanly; audit must be signed off

Completes

• A0 epic — Q3 sign-off is the final gate before tagging v0.1.0.

Sub-Tasks

• Decide and document MSRV (lower bound ≥ 1.65; exact pin TBD)
• Add CI gate: cargo fmt --all -- --check
• Add CI gate: cargo clippy --workspace --all-features
• Add CI gate: cargo test --workspace (no_std, alloc, std)
• Ensure full feature matrix builds run in CI (per C0)
• Make no_std build gate mandatory (fails CI if broken)
• Add CHANGELOG.md / release notes scaffold
• Confirm cargo doc --workspace --all-features builds without warnings
• Confirm examples (if any in core) compile under the feature matrix

Open Questions

• MSRV: exact pin within ≥ 1.65 range — conservative (1.65) for broad
  embedded toolchain compatibility, or newer for better const-generic error
  messages? Decide before any CI pin is added.
• Is the CI runner self-hosted (for embedded cross-compile verification) or
  GitHub-hosted only for v0.1.0?
• Does the no_std gate require a thumbv7em-none-eabi cross-compile check,
  or is a --target build on the host sufficient for v0.1.0?