D567 - Placeholder tagging is opt-in

Status: decided Date: 2026-09-04

Under ORBISTOUN_TAG_PLACEHOLDERS an unimplemented function answers 0x7fff_0000 | (0x10 + its stub slot), so the value names its source; an ordinary run answers 0x7fff_0001. The variable is Effect::Intervenes, tags start at 0x7fff_0010, and a decoded tag counts only when it matches an import the run called. Precedence is override, then tag, then declared return.

Why: a placeholder found in a guest argument is otherwise unattributable, and a finding that sends a reader searching must carry what to look at. Much recorded evidence fixes arities by spotting 0x7fff_0001 in a register, so a new default would stale it at a stroke. The floor keeps the fixed GuestError codes from being read as tags, and the called-import check filters stale register contents that land in the tag range.

Rejected:

• Tagging by default: invalidates every recorded fact that reads the plain placeholder.
• Tags from zero: 0x7fff_0001 would be attributed to whatever import sits at slot 0.
• Decoding any value in range: stale registers produce confident wrong attributions.