Allow local prefix names in Idriç source

[isomorphisms]

Scope

Repair the Idriç parser behavior exposed by dilapidated-shed/icu#13 without rewriting the downstream client.

Reduction

The reported failure at Right headers ⇒ do was a backtracking symptom, not the defective construct. Unicode arrows, do branch bodies, sibling dedents, nested cases, cases inside let/do, multiline application, and the downstream result-handling shape all parse independently.

The minimal failure is:

local_prefix : Integer → IO Integer
local_prefix value = do
  let prefix = value + 1
  pure prefix

prefix was globally tokenized as a fixity keyword, so it could not be used as a local name in .idric source.

Repair

• Reclassify prefix as an identifier only at the .idric source boundary.
• Accept that identifier contextually in a prefix-fixity declaration.
• Reduce edric005 to the local-name regression.
• Leave ordinary .idr tokenization unchanged.

Receipts

• Focused edric005: PASS.
• Full focused Edriç suite (edric001–006, edric009): PASS.
• .idric prefix-fixity declaration: PASS.
• Existing .idr import009 prefix-fixity test: PASS.
• Unchanged dilapidated-shed/icu#13 source: all six package modules compile and the icu executable is produced.
• ICU OpenAI tests: 7/7 PASS.
• ICU transport-model equivalence checks: PASS.

Downstream: dilapidated-shed/icu#13