Phase 2b.3: switch Value::Text to Arc<String> with RC=1 in-place concat

[danieljohnmorris]

Summary

Third (and last) step of the tree-walker RC-aware mutation rollout. Mirror of Phase 2b.1 (Map, PR #261) and Phase 2b.2 (List, PR #273):

• Value::Text(String) becomes Value::Text(Arc<String>) so cloning a string is a refcount bump rather than a full byte copy.
• The existing self-rebind concat peephole in eval_self_rebind_concat (added for Lists in interpreter: RC-aware Value::List with self-rebind append peephole #273) gains a Text branch. When prev and rhs are both Value::Text, Arc::make_mut runs on the refcount=1 Arc and push_str mutates the inner String in place.

The classic string-accumulator loop folds from O(n^2) to O(n) amortised on the tree engine. 5k iterations of s=+s "x" go from 10+ seconds to milliseconds.

Repro

build n:n>n;s="";@i 0..n{s=+s "x"};len s
demo>n;build 5000

• Before: 10+ seconds, every iteration clones the whole String.
• After: a millisecond or two, every iteration uses Arc::make_mut + push_str.

The alias guard inherited from match_self_rebind_concat (which calls expr_refers_to to reject s = s + s) carries over unchanged. Non-rebind shapes (b = +a c) still go through apply_binop so the caller's a is preserved, same as the VM OP_ADD_SS split landed in #260 and the Cranelift jit_concat split landed in #250.

What's in the diff

• interpreter: switch Value::Text to Arc<String> with RC=1 in-place concat (8a05a30) - mechanical rewrite. Every Value::Text(String) construction becomes Value::Text(Arc::new(...)) across the interpreter, VM, Cranelift JIT, json bridge, http/mcp tool providers, and the CLI arg-parser. Every Value::Text(s) read that needs an owned String (MapKey::Text, serde_json::Value::String, parts: Vec, etc.) becomes (**s).clone(). No behavioural change on its own.
• interpreter: eval_stmt peephole for self-rebind text concat (daab1c1) - adds the (Value::Text, Value::Text) branch to eval_self_rebind_concat and updates the match_self_rebind_concat docstring.
• test + example: tree-walker string accumulator regression coverage (0974b0d) - 10 regression tests covering literal / variable / Call-returning-Text rhs / foreach / non-rebind / self-alias / numeric fallthrough / 5k scale, plus examples/string-accumulator-tree.ilo so the engine harness runs the same shapes across tree, VM, and Cranelift.

Test plan

• cargo build --release --features cranelift clean
• cargo test --release --features cranelift - all 112 suites green
• cargo fmt --check clean
• cargo clippy --release --features cranelift --all-targets -- -D warnings clean
• New regression file: 10/10 pass
• Engine harness picks up examples/string-accumulator-tree.ilo across tree, VM, Cranelift
• 5k-iteration scale check completes well under the 5s ceiling

Follow-ups

None. This closes Phase 2b. Map (#261), List (#273), and Text (this PR) all use the same Arc + self-rebind peephole shape on the tree engine, matching the VM OP_* rebind splits and the Cranelift jit_* rebind splits that landed earlier.

No README / SPEC updates: this is a runtime-internal perf fix with no user-observable semantic change beyond the formerly O(n^2) accumulator going O(n).

[codecov]

Codecov Report

❌ Patch coverage is 96.42325% with 24 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/interpreter/mod.rs	95.20%	23 Missing ⚠️
src/vm/jit_cranelift.rs	99.28%	1 Missing ⚠️

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