perf: fast path len identifier calls

[larimonious]

Summary

• Adds a targeted interpreter fast path for len(identifier) so large arrays/maps are not cloned just to compute length.
• Keeps normal len shadowing behavior intact; user-defined len still wins.
• Adds a /native/calls benchmark fixture and records the DD-061 PR 5 result/recommendation update.

Benchmark

Local 3s/3-run wrk pass at 16 connections / 2 threads, comparing origin/main plus the new benchmark fixture to this branch's dev-release binary:

• /native/calls: 349.95 -> 1574.70 RPS (+350.0%)
• other routes stayed within normal local noise

Verification

• cargo fmt
• cargo build --profile dev-release
• cargo test
• ./target/dev-release/ntnt docs --generate
• cargo fmt -- --check
• git diff --check
• ./target/dev-release/ntnt lint examples/perf/server.tnt — 0 errors / 0 warnings; existing benchmark-file contract suggestions only
• focused rerun after final simplification: cargo test fast_path --lib, cargo test environment_get --lib

[greptile-apps]

Greptile Summary

This PR adds a targeted fast path for len(identifier) calls in the interpreter, avoiding unnecessary cloning of large arrays and maps just to compute their length, while preserving full user-defined shadowing semantics. It also converts Environment::get from recursive to iterative to prevent stack overflow on deep scope chains.

• Fast path (src/interpreter.rs): Before the normal call dispatch, if the callee is len, the sole argument is an identifier, and len currently resolves to the builtin NativeFunction, len_of_binding is called directly on the environment chain — identical logic to the builtin, no clone. Falls through to the normal path for any expression argument or when len is shadowed.
• Iterative get: The existing recursive Environment::get is rewritten as an iterative loop with a 64-depth stress test added to guard against regression.
• Benchmark fixture: A new /native/calls route and benchmark entry exercise the fast path in a 5000-iteration loop, recording a +350% RPS gain on local hardware.

Confidence Score: 5/5

Safe to merge; the fast path is strictly additive and falls through to the existing normal dispatch on any non-matching case.

The fast path's len_of_value is byte-for-byte identical to the existing builtin's match arms. Shadowing, type errors, and parent-scope traversal are all covered by new unit tests. The iterative get rewrite is tested with a 64-level depth chain. No behavioral changes are possible for callers that do not hit the narrow len(identifier) pattern.

No files require special attention.

Important Files Changed

Filename	Overview
src/interpreter.rs	Adds iterative get, len_of_binding, and len_of_value to Environment, and inserts a fast path for len(identifier) in expression evaluation; fast-path logic matches the existing builtin exactly; tests cover shadowing, type errors, and grandparent-scope traversal.
examples/perf/server.tnt	Adds /native/calls benchmark route that exercises len(identifier) in a 5000-iteration loop across string, array, and expression arguments.
scripts/bench/run-benchmarks.py	Registers the new /native/calls endpoint in the benchmark suite; mechanical, low-risk change.
examples/perf/README.md	Documents the new /native/calls route in the benchmark fixture table.
design-docs/dd-061-interpreter-performance-roadmap.md	Updates PR 5 scope from broad native-call fast path to targeted len(identifier) fast path, marks acceptance criteria complete, and records benchmark results.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Expression::Call evaluated"] --> B{"function is Identifier?"}
    B -- No --> Z["Normal call dispatch"]
    B -- Yes --> C{"name is len AND 1 argument?"}
    C -- No --> D{"name is old?"}
    D -- Yes --> OLD["old() postcondition handling"]
    D -- No --> Z
    C -- Yes --> E{"argument is Identifier?"}
    E -- No --> Z
    E -- Yes --> F["borrow env, get len"]
    F --> G{"resolves to NativeFunction named len?"}
    G -- No --> Z
    G -- Yes --> H["borrow env, len_of_binding"]
    H --> I{"binding found?"}
    I -- No --> Z
    I -- Yes --> J["len_of_value without clone"]
    J --> K{"value type?"}
    K -- String --> L["Int s.len()"]
    K -- Array --> M["Int a.len()"]
    K -- Map --> N["Int m.len()"]
    K -- Other --> O["TypeError"]
    Z --> P["eval function expr, clone arg, NativeFunction dispatch"]

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flowchart TD
    A["Expression::Call evaluated"] --> B{"function is Identifier?"}
    B -- No --> Z["Normal call dispatch"]
    B -- Yes --> C{"name is len AND 1 argument?"}
    C -- No --> D{"name is old?"}
    D -- Yes --> OLD["old() postcondition handling"]
    D -- No --> Z
    C -- Yes --> E{"argument is Identifier?"}
    E -- No --> Z
    E -- Yes --> F["borrow env, get len"]
    F --> G{"resolves to NativeFunction named len?"}
    G -- No --> Z
    G -- Yes --> H["borrow env, len_of_binding"]
    H --> I{"binding found?"}
    I -- No --> Z
    I -- Yes --> J["len_of_value without clone"]
    J --> K{"value type?"}
    K -- String --> L["Int s.len()"]
    K -- Array --> M["Int a.len()"]
    K -- Map --> N["Int m.len()"]
    K -- Other --> O["TypeError"]
    Z --> P["eval function expr, clone arg, NativeFunction dispatch"]

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_{Reviews (2): Last reviewed commit: "test: cover len fast path parent scopes" | Re-trigger Greptile}