fix: route legacy-usearch notices to stderr for clean structured output

[DxTa]

Problem

--format json output was polluted by diagnostic notices (Detected legacy usearch index...) printed to stdout via the Rich console. This broke JSON consumers — e.g. the pi sia-code-proactive extension's JSON.parse hit the leading warning text and fell back to dumping the raw stdout (warnings + pretty JSON) as the tool result, adding ~580 chars of ANSI warnings and defeating compact/projection formatting.

Root cause

create_backend() in cli.py and runtime_context.py emitted the legacy-usearch compatibility notices with console.print(...) (stdout), even in structured output mode.

Fix

Route these notices to a stderr Rich console (err_console = Console(stderr=True, ...)):

• stdout stays pure JSON for --format json / machine consumption.
• stderr still shows the notices (with color) in the terminal — no UX regression for interactive use.
• The existing suppress_stdout_notices guard is preserved unchanged.

Files

• sia_code/cli.py — add err_console, 2 notice blocks → err_console.print
• sia_code/runtime_context.py — same (duplicate backend-resolution path)

Verification

$ sia-code search --format json -k 5 -- "zsh alias" 2>err.txt > out.txt
$ head -1 out.txt
{
$ python -c "import json; print(len(json.load(open('out.txt'))['results']))"
5
$ head -3 err.txt   # notices now on stderr
Detected legacy usearch index with implicit storage backend. Using legacy backend for compatibility.
Set 'storage.backend=sqlite-vec' and run 'sia-code index --clean .' to migrate when ready.

Diff: +10/-8 across 2 files. Imports verified.

Notes

• Complementary root-cause option (not in this PR): migrate the index with sia-code index --clean . to sqlite-vec, which eliminates the legacy notices entirely.
• The pi extension also gained a belt-and-suspenders prefix-strip (maybeJson finds the first {), so older CLI versions that still print notices to stdout won't break parsing.