feat(Attributes): Add unified [<Assignment>]; obsolete the six legacy

[dimension-zero]

feat(Attributes): Add unified []; obsolete the six legacy

• Attributes.fs: New AssignmentAttribute(separator, ?allowSpaced)
  subsumes CustomAssignment / EqualsAssignment / ColonAssignment /
  CustomAssignmentOrSpaced / EqualsAssignmentOrSpaced /
  ColonAssignmentOrSpaced. The legacy six are now marked []
  with a deprecation message pointing at the equivalent
  [<Assignment(...)>] invocation. Behaviour and storage layout are
  preserved.
• Attributes.fs: Add a file-scoped #nowarn 44 so the legacy attributes
  that derive from now-obsolete bases (EqualsAssignmentAttribute :>
  CustomAssignmentAttribute, etc.) still compile inside this file.
  Callers outside the file still see the obsolete warning.
• PreCompute.fs customAssignmentSeparator: read AssignmentAttribute
  first; if present it wins. Mixing the new attribute with any of the
  legacy six is rejected with a clear error so we don't have to invent
  a merge policy. The legacy two-attribute matrix is preserved
  byte-identical for callers still using it.

Strictly additive at the binary level; existing code emits a
deprecation warning only.

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test(AssignmentAttribute): Coverage for unified [] attr

7 new tests:

• [<Assignment("=")>] parses --port=8080 and rejects spaced --port 8080.
• [<Assignment("=")>] result matches the legacy []
  parity case.
• [<Assignment(":", allowSpaced = true)>] accepts both --tag:value and
  --tag value spaced forms.
• Parity with [].
• Mixing [] with the legacy [] on the same
  case raises a clear ArguException whose message mentions Assignment —
  the rejection prevents accidental double-attribution.

The file uses #nowarn 44 because it deliberately exercises the obsolete
legacy attributes alongside the new one. Net suite on this branch:
112 -> 119.

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