Configurable canonical priority range (--priority-range)

[stackedsax]

What

Make the canonical priority band a parameter instead of a hardcoded 0–1000.

Every scheduler's native priority is normalized onto a common interchange band
and denormalized back on emit. That band was fixed at 0–1000, which caps
round-trip precision: PBS's 2048-value native range can't survive a pass through
a 1001-value band.

• splat.CanonicalScale (default {0, 1000}) is the band Normalize/
  Denormalize now map through.
• bammm convert --priority-range MIN:MAX sets it per run (default 0:1000).
  splat.ParseRange validates the value (MIN:MAX, MAX > MIN).

Goal: round-trip precision. Widening the band recovers fidelity for
schedulers with large native ranges; the default behavior is unchanged.

Demo

# source: #PBS -p 500
pbs -> pbs, default 0:1000   -> #PBS -p 501   (lossy)
pbs -> pbs, --priority-range 0:100000 -> #PBS -p 500   (exact)

(A Slurm source shows no change from widening — Slurm's native 0–1000 range is
already coarser than the band, so there's no lost precision to recover. The
benefit applies when the source range is finer than the band.)

Testing

• TestParseRange: valid/invalid flag values.
• TestPriority_WideCanonicalImprovesRoundTrip: the default band loses
  precision across PBS's full range; a band ≥ the native range is lossless.
• TestConvert_PriorityRangeFlag: the flag is validated and applied
  end-to-end; omitting it restores 0–1000.
• Full suite, go vet, gofmt clean. SPEC.md updated.

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