codegen: store singular message fields inline by default (PointerRepr::Inline) (#248)

[iainmcgin]

Closes #248.

Summary

Makes inline storage the default for singular message fields: codegen now emits MessageField<T, ::buffa::Inline<T>> (laid out as Option<T>, no per-field heap allocation) for every non-recursive field. Recursive fields are detected at codegen time and stay on Box automatically, so the default is always sized.

• buffa::Inline<T> — a #[repr(transparent)] ProtoBox<T> newtype that stores T directly.
• PointerRepr::Inline — now #[default]. The recursion DFS (inline_is_recursive) follows both inline-edge kinds — rule-matched unbox_oneof variants and Inline-resolved singular fields — so a cycle through one of each is caught for both knobs. ctx.pointer_repr() demotes any Inline not in the precomputed safe set to Box; oneof-variant paths are never in that set, so a blanket Inline rule can't bypass unbox_oneof's guard.
• Opt-out is the existing box_type_in(PointerRepr::Box, &[".pkg.Msg.field"]) (per field/prefix) or box_type(PointerRepr::Box) (restore the old global default). box_type_in now normalizes a missing leading dot — previously a dotless path silently matched nothing.

Benchmark validation

TriMesh is a new benchmark message (256 Face elements per payload, four singular Vertex submessages each — the workload shape where boxed storage costs four heap allocations per element). On bare-metal (c7i.metal-24xl, 10s measurement), inline vs box:

op	mesh (target)	analytics_event (control)
decode	−36.3% (2.08 → 1.32 ms)	+4.5% (p=0.44, noise)
merge	−55.9%	−3.8% (noise floor)
decode_view	−15.4%	−1.9%
json_decode	−11.0%	−0.4%
encode / compute_size / json_encode	flat	flat

AnalyticsEvent has zero singular message fields (everything message-typed is repeated), so its flat result confirms the change doesn't perturb messages it can't help.

Fallout

• json_helpers::message_field_always_present and pool::clone_options genericized over P: ProtoBox<T> — the only two runtime helpers that hardcoded the Box pointer.
• Bootstrap descriptor types regenerated (*Options fields now inline). WKTs are byte-identical — none has a singular message field.
• DESIGN.md, docs/guide.md, and the MessageField/ProtoBox docs updated for the inline default.

Breaking change

Generated singular message field types change from MessageField<T> to MessageField<T, Inline<T>>. Explicit MessageField<Foo> annotations now mean the boxed form and will mismatch — drop the annotation and let P infer from the field's declared type. See the changelog entry for the full migration note.

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