docs+lint: safer forms are not semantic equivalents

[Kiran01bm]

Summary

CREATE INDEX and CREATE INDEX CONCURRENTLY converge on the same declared end state but
are not semantic equivalents — different locking, transactionality, and failure modes.
This PR sweeps docs, API comments, and CLI output so every safer-rewrite recommendation
reads as an advisory safer form the engine owns executing, never as an equivalent or an
instruction to run manually. No behavior change.

What

• docs/design-principles.md, docs/high-level-design.md, docs/low-level-design.md:
  replace "native equivalent" / "safer equivalent" with "safer native form" and state the
  operational differences (a failed CONCURRENTLY build leaves an INVALID index the
  executor must detect via pg_index.indisvalid and recover); the HLD advisory diagram now
  says safer form: instead of run instead:.
• pkg/planner (Decision.SaferSQL) and pkg/lint (CodeBlockingIdiom,
  Finding.Suggestion) doc comments carry the same caveat.
• CLI output: lint prints safer form (not equivalent — see docs/postgres-online-ddl-reference.md); diff dry-run prints safer form the engine would run (not equivalent — …) instead of the engine would run instead:.

Why

A recommendation that prints "run instead: CREATE INDEX CONCURRENTLY …" invites an operator
to paste it into psql — bypassing the engine's execution-time guards (invalid-index
detection and recovery) while implying identical semantics. The distinction is declarative
vs. operational: from a desired-state diff standpoint the end state is identical
(CONCURRENTLY is not catalog state), but the execution paths fail differently, and only
the engine owns the recovery. Wording everywhere now matches the execution-safety model in
docs/postgres-online-ddl-reference.md.

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