When hrr_dim=1 the prefixed float32 blob (4+4=8 bytes) collides in
size with a raw float64 blob (1×8=8 bytes), making the format
discriminator in bytes_to_phases ambiguous — a legacy blob starting
with HRR1 would be misread as a prefixed float32 vector.
- phases_to_bytes now accepts an optional dim and falls back to
writing raw float64 when the two blob sizes are equal.
- bytes_to_phases prefers the legacy float64 interpretation when
sizes collide and dim is provided, since phases_to_bytes never
writes a prefixed float32 blob at dim=1.
- Three regression tests cover dim=1 write, round-trip, and the
legacy-prefix collision case.
Addresses hermes-sweeper review on PR #30499.
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