perf(memory): store holographic vectors as float32 (#30499 salvage)

[kshitijk4poor]

Salvage of #30499 by @JabberELF — both commits cherry-picked to preserve authorship; one conflict resolved against current main.

Context — what this changes for users

The holographic memory plugin stores one HRR phase vector per fact as raw float64 bytes — 8KB per fact at the default dim=1024. Phase angles don't need float64: float32's ~1e-7 relative error is far below the similarity noise floor. This PR halves vector storage (4KB/fact; a 10k-fact store shrinks ~40MB) with a versioned format: new blobs carry an HRR1 magic prefix, and the decoder promotes everything to float64 on read so downstream math is unchanged.

Migration story (the risky part — independently audited)

• Existing float64 blobs (no prefix) still load: decoder detects HRR1 + exact float32 size, falls back to raw float64 by length. Traced end-to-end with a real legacy blob written into an existing DB — reads fine, no crash, no garbage.
• dim=1 size-collision ambiguity (8 bytes = 1 float64 = prefix+1 float32) explicitly handled: writer falls back to raw float64 when sizes collide; reader prefers the legacy interpretation. Both directions test-pinned, including the adversarial HRR1-prefix-collision case (second commit).
• Mixed-precision safety: decode always promotes to float64, so similarity() never compares mixed dtypes; all 8 production call sites pass dim=self.hrr_dim.
• Round-trip parity tests at atol=1e-6 (the float32 quantization bound).

Salvage notes

• One conflict in retrieval.py against current main: upstream had since landed the perf(memory): hoist loop-invariant HRR encodes out of retrieval loops (#76142 salvage) #76881 loop-invariant encode hoists. Resolved by keeping the hoisted shape and applying this PR's dim-passing onto it. All other hunks applied clean.

Verification

• 245 tests green (new storage suite + full tests/plugins/memory/), ruff clean.
• Mutation check: production reverted → 9/11 new tests fail (the 2 legacy-read tests correctly still pass); restored → 11/11.

Closes #30499.