phase29ab(p2): add seg trim fixture+smoke and docs

src/mir/builder/control_flow/joinir/patterns/pattern2/README.md

@@ -10,6 +10,11 @@
 - Supported: A-3 Trim / A-4 DigitPos (promote LoopBodyLocal to carrier)
 - ConditionOnly carriers are recalculated per iteration (no host binding)
 
+## Trim (seg) minimal shape
+- Example shape (A-3): `local seg = s.substring(i, i + 1)`
+- Break guard: `if seg == " " || seg == "\\t" { break }`
+- seg is read-only (no reassignment in the loop body)
+
 ## Carrier binding rules (Pattern2)
 - `CarrierInit::FromHost` -> host binding required
 - `CarrierInit::BoolConst(_)` / `CarrierInit::LoopLocalZero` -> host binding is skipped
@@ -19,6 +24,7 @@
 - multiple breaks / continue / return in the loop body
 - reassigned LoopBodyLocal or ReadOnlySlot contract violations
 - break conditions with unsupported AST shapes
+- seg reassignment or non-substring init (e.g., `seg = other_call()`)
 
 ## Fail-Fast policy
 - `PromoteDecision::Freeze` -> Err (missing implementation or contract violation)