Summary

Two related cleanups in one PR:

• REF-6 cascade: LoggingInstrument and OpenTelemetryInstrument cached mutable runtime state via object.__setattr__ workarounds because they were declared frozen=True. Python's dataclass rules forbid surgically dropping frozen=True from one subclass while the parent stays frozen — the cascade is required. BaseInstrument and all 22 instrument subclasses lose frozen=True. The 4 object.__setattr__ call sites in LoggingInstrument and OpenTelemetryInstrument become plain self._x = ... assignments. Configs all keep frozen=True — only instruments lose immutability.
• LOW-4: FastAPIConfig.application used default=None with a # ty: ignore[invalid-assignment]. Replaced with a typed sentinel _UNSET_FASTAPI_APP: typing.Final = typing.cast("fastapi.FastAPI", object()). The __post_init__ check changes from if not self.application: to if self.application is _UNSET_FASTAPI_APP:. Added an explicit None guard with ConfigurationError to prevent silent AttributeError if a caller explicitly passes None (was a tolerated type-violation pre-change; now fails cleanly).

12 files modified; 24 dataclass declarations lose frozen=True; 4 object.__setattr__ call sites simplified; 1 # ty: ignore removed; 1 explicit-None guard added.

No behavior change for typed callers. 128/128 tests pass.

Closes REF-6 and LOW-4 from an internal audit.

Why the cascade

Python's @dataclasses.dataclass enforces that a non-frozen dataclass cannot inherit from a frozen one (and vice versa) — TypeError at class definition. To drop frozen=True from LoggingInstrument (REF-6's stated target), BaseInstrument must also drop it, which propagates to every other instrument subclass. The sequencing spec's PR13 section called for a surgical 2-class change; the actual cascade is 24 classes.

Test plan

• just test — 128/128.
• just lint — clean (no # ty: ignore left in FastAPIConfig).
• grep -n "frozen=True" lite_bootstrap/instruments/ lite_bootstrap/bootstrappers/ — only config classes match.
• Reviewer: confirm LoggingInstrument's and OpenTelemetryInstrument's direct assignments preserve the try/finally exception safety from PR3.

Notes for reviewer

• Pyright now flags bootstrap_config override invariance in framework subclasses (e.g., FastAPICorsInstrument's bootstrap_config: FastAPIConfig overriding CorsInstrument's bootstrap_config: CorsConfig). This is a Pyright-only concern about mutable-field covariance; ty check passes clean. Worth a separate look later if the team cares about Pyright; out of scope for this PR.
• The explicit-None guard in FastAPIConfig.__post_init__ is added in response to code review — it catches the new behavioral edge case where application=None would otherwise hit AttributeError instead of building a fresh app like the pre-PR truthiness check did.

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