Fix sgpd entry boundaries and v3 encoding invariants

[kixelated]

Closes #190.

What changed

• Decode unknown sample-group entries using their declared description_length, preserving boundaries between multiple entries.
• Require the default fields mandated by the selected v2/v3 sgpd encoding instead of emitting a malformed box.
• Add regression coverage for multiple unknown entries and incomplete essential/v3 values.

Root cause and impact

Unknown grouping types previously decoded the entire remaining box payload into the first entry, corrupting the parsed entry structure. Separately, setting essential without v3 defaults selected version 3 but omitted required fields, producing bytes the decoder could not read back.

Validation

• just check
• just test (238 tests passed)
• git diff --check