docs: MySQL comparison, three-bucket cost model, binary-coercible type changes

[Kiran01bm]

Doc-only: adds the MySQL ↔ PostgreSQL comparison reference the docs already cite, a three-bucket cost model on top of the online-DDL reference, and a deep-dive on when ALTER COLUMN TYPE is a relabel versus a rewrite.

Why

Six places in docs/ link to a mysql-vs-postgresql.md that never existed in this repo, so the lock-queue explanation and the Spirit → PostgreSQL primitive mapping they lean on were unreachable. Separately, the online-DDL reference lists two axes (lock level, rewrite/scan) but never states the synthesis readers actually need — that every PostgreSQL DDL falls into one of three buckets that correspond to MySQL's INSTANT / INPLACE / COPY — and the binary-coercible verdict had a one-line definition with no account of how PostgreSQL decides or which changes only look free.

What

• docs/mysql-vs-postgresql.md — new: how each engine expresses online DDL, lock-mode → MDL mapping, why DDL is dangerous (the lock-queue pile-up, same in both engines), Spirit → PostgreSQL primitive mapping.
• docs/postgres-online-ddl-reference.md — new "three buckets" section (catalog-only ≈ INSTANT, full scan no rewrite ≈ INPLACE, full rewrite ≈ COPY) tied to the plan-report reason vocabulary; dangling refs turned into links.
• docs/binary-coercible-type-changes.md — new: the structural rewrite test in ATColumnChangeRequiresRewrite, the four shapes that skip a rewrite and why, six categories of forced rewrite with examples, "looks free but rewrites" (shortening varchar, numeric scale/precision with row-level before/after, char(n) → text, arrays, non-UTC timestamp ↔ timestamptz), what "no rewrite" still costs, catalog/filenode/event-trigger checks, and the exact rules the planner's binaryCoercible accepts.
• Dangling 12-mysql-vs-postgresql.md / bare mysql-vs-postgresql references in design-principles.md, low-level-design.md, high-level-design.md, invariants.md now link to the real sections; docs/README.md table updated.

No code, contract, or capability changes.

Before / after

Before
  design-principles.md ─┐
  low-level-design.md  ─┤──▶ "12-mysql-vs-postgresql.md § ..."   (404 — file never existed)
  invariants.md        ─┤
  online-ddl-reference ─┘
  online-ddl-reference: axis 1 (lock) + axis 2 (rewrite/scan) ... no synthesis
  plan-report reasons:  metadata-only | binary-coercible | safer-idiom | type-rewrite ...
                        (no doc says which bucket each belongs to)

After
  design-principles.md ─┐
  low-level-design.md  ─┤──▶ mysql-vs-postgresql.md ──▶ § lock queue, § primitive mapping
  invariants.md        ─┤
  online-ddl-reference ─┘
  online-ddl-reference: ┌────────────────┬──────────────────────┬──────────────┐
                        │ catalog-only   │ full scan, no rewrite│ full rewrite │
                        │ ≈ INSTANT      │ ≈ INPLACE            │ ≈ COPY       │
                        │ native as-is   │ safer sequence       │ copy-and-swap│
                        └───────┬────────┴──────────────────────┴──────────────┘
                                └──▶ binary-coercible-type-changes.md
                                     (how PG decides; four free shapes;
                                      six rewrite categories; looks-free traps)

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