Push Flow idiomatics across the Euler corpus

[godofecht]

Continues the direction from PR #10 and its review comment by turning repeated corpus patterns into actual shared conventions.

This pass:

• expands euler.digits beyond the original p016 exemplar with reset, integer loading, reverse-add, and palindrome operations
• migrates p020, p055, and p056 onto the shared representation
• removes full-capacity digit clearing in p056 by treating len as the live-prefix invariant
• finishes the p054 idiom pass by converting its fixed five-card loops to range for while retaining descending/data-dependent loops
• converts fixed-bound loops in p055, p056, p072, p089, and p601 to range for while keeping data-dependent cursors and carry loops as while
• migrates exact number-theory duplicates to euler.nt in p601 and p570 while deliberately retaining p570's local i128 modular exponentiation because its overflow semantics differ
• adds FLOW_IDIOMS.md, documenting corpus-derived adoption rules and concrete language/library candidates such as lexical resource ownership and fixed-array initialization
• links the idiom guide and shared lib/ convention from the README so the practice is discoverable

The intent is not to maximize use of language features. An idiom is adopted when it recurs, reduces duplicated protocol/state, and preserves the same low-level execution shape.

Qualification is local-only. Keep this PR draft until the complete C-backend corpus and any intended MLIR parity checks have been run locally against the exact head SHA and the results recorded here. Hosted workflow state is not release or merge evidence for this branch.

Branch status: flow-idiomatics-corpus-pass is synchronized with current main at head d50e35cf131485e648789ebe75636c1047ac732c, is 0 commits behind, and is technically mergeable. Mark ready or merge only after exact-head local qualification is recorded.

Follow-up to #10.

[godofecht]

Maintenance update (26 Aug 2026): the branch is now synchronized with current main at head d50e35cf131485e648789ebe75636c1047ac732c and is technically mergeable. Hosted workflow state is no longer qualification evidence for this PR and no workflow runs should be triggered for it. Keep the PR in draft until the complete C-backend corpus and any intended MLIR parity checks have been run locally against this exact head SHA and the results are recorded here.