refactor(regression): collapse path-placeholder clump into PlaceholderMap

[lewisjared]

Description

Behaviour-preserving refactor stacked on #756. It collapses the regression-test path-placeholder data clump into a single PlaceholderMap value object.

Previously the placeholder set (<OUTPUT_DIR> / <TEST_DATA_DIR> / <SOFTWARE_ROOT_DIR>) was expressed three ways: loose output_dir / test_data_dir / software_root_dir parameters threaded through ~8 signatures, a private _placeholder_pairs list, and hand-built {real: token} dicts at the verification call sites. Adding the third placeholder (<SOFTWARE_ROOT_DIR>, in #756) had to be threaded through every layer, and the verification side had already drifted — it only knew two of the three tokens.

PlaceholderMap (in climate_ref_core.output_files) now owns the (token, directory) set. The configuration-stable tokens come from PlaceholderMap.for_baseline(...); the per-execution <OUTPUT_DIR> is late-bound via .with_output(...); and .sanitise() / .hydrate() / .as_replacements() replace the old free functions. The three free functions (to_placeholders, from_placeholders, _placeholder_pairs) are deleted, and every caller now passes a single placeholders argument. The verification side (RegressionValidator, plus the conftest ExecutionRegression / DiagnosticValidator) derives its replacements from the same map, so capture and verification can no longer declare different token sets and drift apart.

No behaviour change: the path rewriting, longest-match-first ordering, and float/provenance handling are identical. Verified with ruff, ruff format, mypy, and the existing unit + integration suites (241 passed, 1 skipped), plus an independent review pass.

Stacked on #756 — until that merges, this PR's base is feat/pmp-baselines, so review it on top of #756 (or retarget to main once #756 lands; the diff stays refactor-only).

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/ (759.trivial.md)

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