adoptLegacyFile abstract method

Migration-only (install-identity-namespacing, 2026-08-02): if a file already exists on disk under oldFilename (the pre-refactor flat name) and nothing exists yet under newFilename (the namespaced identity), renames it in place and returns true — the caller should treat this as "already installed" and persist repository metadata under newFilename instead of re-downloading. Returns false when there is nothing to adopt: no old file, newFilename already exists, or oldFilename == newFilename (nothing changed identity, so there is nothing to migrate).

SAFE ONLY when the caller KNOWS the file under oldFilename belongs to the model it is adopting it into — the old flat name carries no type marker, so a blind adoption could hand this model another model's file and resurrect the exact collision this refactor fixes. Two call patterns satisfy that precondition:

1. The old basename is UNIQUE across the whole model catalog (e.g. a TTS bundle's .tflite graphs, or a TTS basename gated on TtsModelType-catalog uniqueness in TtsInstallationBuilder) — a unique name can only have been written by the one model that owns it.
2. The caller is migrating a SINGLE KNOWN active model (the active-model restore paths in MobileModelManager/WebModelManager): the model's own id disambiguates the owner even for a colliding-basename companion (tokenizer.json, sentencepiece.model), because the pre-refactor collision meant only the last-installed = active model's file could exist under the plain name. NEVER call this for a colliding-basename companion at plain INSTALL time (where the on-disk plain file could belong to any previously-installed model). See docs/superpowers/specs/2026-08-02-install-identity-namespacing-design.md.