docs: prepare 1.5.0 release notes (README pairing, CHANGELOG date)

[luisleo526]

Release-prep docs for 1.5.0, before the release dispatch (the release workflow publishes the README as the PyPI page at the tag). Docs only: no VERSION bump (the workflow does that), no gitlink, no code.

• README: a 1.5.0 row in the engine pairing table (codegen 1.5.0 with engine v1.5.0, 2026-10-12); every line that named 1.4.0 as current now names 1.5.0; the "on main / planned" wording for 1.5.0 removed; the source link, clone --branch and the engine-pair section point at v1.5.0.
• CHANGELOG: ## 1.5.0 — 2026-10-12; only the section's intro paragraph is rewritten to describe the release (paired with engine v1.5.0); the rest of the 1.5.0 entry is unchanged.
• docs/PUBLIC_CONTRACT.md: "This revision includes the 1.5.0 pair (2026-10-12)", the same pattern as the 1.4.0 dating commit 721a4ea; the "planned number" sentence and a stale "not yet released" availability line removed.

Left on purpose: the 1.4.0 validation sentence in the README (history, still true), the facts-rendered results banner (the 1.5.0 facts render decides it), and the @pf-trace release list (it names no current release).

Every hunk was read by the supervisor; a grep shows no line still describing 1.5.0 as planned or unreleased.

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