Python: Add f-string support for Python template/pattern API

[knutwannheden]

Summary

Add f-string and t-string support for the Python template/pattern API, enabling a more natural syntax for defining captures in templates.

Three ways to create templates with captures:

expr = capture('expr')

# 1. Plain string with explicit kwargs (existing)
tmpl = template("print({expr})", expr=expr)

# 2. f-string (new) — no name duplication
tmpl = template(f"print({expr})")

# 3. t-string (Python 3.14+, new) — structured interpolation
tmpl = template(t"print({expr})")

All three paths produce identical results. RawCode (via raw()) can be mixed with captures in f-strings and t-strings for splice-time code insertion (e.g. dynamic method names).

Implementation

• Capture.__format__ emits the internal placeholder identifier (__plh_name__) directly, making f-string code already in the engine's internal format — substitute_placeholders becomes a no-op
• convert_tstring does the same for t-strings
• Thread-safe auto-registration via contextvars.ContextVar prevents interference between concurrent f-string evaluations
• Stale registry entries are always cleared to prevent leaking between calls
• Shared resolve_captures() helper dispatches t-string / f-string / explicit-kwargs uniformly for both template() and pattern()

Test plan

• test_capture.py — __format__ output, auto-registration, stale cleanup, kwargs override
• test_tstring_support.py — t-string conversion with captures, raw code, mixed usage
• test_placeholder.py — placeholder conversion, substitution, validation
• test_engine.py — parsing with placeholders, caching, auto-format integration
• test_replacement.py — placeholder replacement, auto-parenthesization