fix(build): lite-scons harness JSON protocol is corrupted by user-script print() #945

Context

lite_scons_harness.py communicates its result to script_runtime.rs by writing JSON to stdout (json.dump(result, sys.stdout), line ~742). But user extra_scripts are exec'd in-process, so any direct print() in a user script lands on the same stdout ahead of the JSON. script_runtime.rs:146 then fails with:

build error: build failed: failed to parse extra_scripts runtime output: expected value at line 1 column 1

Found while standing up the #942 profiling harness: NightDriverStrip env:demo inherits extra_scripts = pre:tools/install_intelhex.py, pre:tools/pio_audit.py, pre:tools/bake_site.py, post:tools/merge_image.py, and both pio_audit.py and bake_site.py print progress banners (>>> NightDriver Coding Standard Audits, Baked index.html -> index.html.gz: 2693 B) straight to stdout. Reproduced by invoking the harness directly with those scripts — stdout begins with the banners, rc=0, JSON unparseable. Printing from PlatformIO extra_scripts is extremely common, so this breaks many real projects, not just NightDriverStrip.

Proposal

In lite_scons_harness.py, capture the real stdout handle at startup and exec user scripts (and Builder/Action callables) with sys.stdout/sys.stderr redirected to buffers. Surface captured text in the result payload (e.g. per-script stdout/stderr fields in lite_scons_records or notes) so output is preserved for verbose logs instead of discarded. Emit the final JSON to the saved real stdout only.

Acceptance criteria

• A user extra_script that calls print() at import time and inside SCons callbacks no longer breaks overlay parsing.
• Captured script output is present in the harness result payload (not silently dropped).
• Rust-side test in script_runtime_tests.rs (or the lite_scons acceptance suite) covering a printing script.
• NightDriverStrip env:demo extra_scripts evaluation succeeds (validated in the meta: profiling-driven build performance burndown (Docker Linux, NightDriverStrip, cold vs hot cache) #942 Docker harness).

Decisions

• Severity: HIGH — user-facing build failure on common real-world projects.
• Fix location: harness-side redirect, not Rust-side "skip garbage until {" — the JSON boundary must stay unambiguous.

Part of #942 (blocks its Phase 1 baseline: the benchmark workload cannot build without this).