Ungate procedures from the unstable feature

[cloutiertyler]

Description of Changes

Graduate procedures (and the outgoing HTTP client used from procedures) out of the unstable feature gate / SPACETIMEDB_UNSTABLE_FEATURES across all module libraries. The procedure macro, ProcedureContext, with_tx/try_with_tx, scheduled procedures, and ctx.http are now available without opting into unstable.

Still gated (unchanged): HTTP handlers/webhooks, views, RLS / client_visibility_filter, and immediate-scheduling (volatile_nonatomic_schedule_immediate).

Per library:

• bindings-sys: ungate the procedure host-call module + raw ABI imports (procedure_start/commit/abort_mut_tx, procedure_http_request) and call_no_ret. Scheduling ABI stays gated.
• bindings (Rust): ungate the procedure macro, ProcedureContext, TxContext/with_tx/try_with_tx, db_read_only/get_read_only, the procedure traits, register_procedure, __call_procedure__, and procedure RNG. The http module (previously gated as a unit) now has fine-grained gating so the outgoing HttpClient is exposed while HandlerContext/Router/handler macros stay gated.
• bindings-csharp: drop [Experimental("STDB_UNSTABLE")] from ProcedureContext.WithTx/TryWithTx (runtime + codegen) and the generated ProcedureTxContext; FFI snapshots regenerated. Handler context members + RLS attribute stay gated.
• bindings-cpp: ungate the procedure ABI (abi.h/FFI.h), tx_execution.h, the outgoing HTTP client (http.h/http_convert.h), and procedure_context.h. handler_context.h/router.h/http_handler_macros.h still #error without SPACETIMEDB_UNSTABLE_FEATURES.
• docs: remove the procedures beta notices (HTTP handlers stay marked beta for a later release); regenerate static/llms.md.
• sdk-test-procedure: no longer needs features = ["unstable"].

API and ABI breaking changes

Not breaking. This is purely additive to the stable surface: procedures become available without the unstable feature, while modules that already opt into unstable are unaffected. The wasm host-ABI imports were already implemented host-side and merely gated module-side, so there is no ABI version change. (No breaking-change label needed.)

Expected complexity level and risk

3/5. The diff itself is mostly removing #[cfg]/#ifdef/[Experimental] guards, but the gate bundled procedures + outgoing HTTP + handlers + the ABI crate together, so the work was in separating procedures from the features that stay gated. Areas reviewers may want to scrutinize:

• The Rust http module went from "gated as a whole" to fine-grained per-item gating; the outgoing HttpClient is exposed while handler types/macros stay behind unstable.
• The unstable cut reaches the ABI crate (bindings-sys), which is the only way procedures can compile without the feature.
• C++ separation of the outgoing HTTP client from HTTP handlers across several headers.

Testing

• Rust: cargo check -p spacetimedb passes in default, --features unstable, and --no-default-features --features unstable.
• Rust end-to-end: sdk-test-procedure (uses procedures, with_tx/try_with_tx, ctx.http.get, new_uuid_v7, scheduled procedures) builds without unstable.
• C#: Codegen + Runtime build; Codegen.Tests pass (6/6) after FFI snapshot regen.
• C++: host syntax-check confirms procedures + ctx.http.send() compile without the flag, full headers compile with it, and handler_context.h still #errors without it. (Not built for the real wasm/emscripten target locally — relying on CI.)
• cargo ci lint components reproduced locally: rustfmt, clippy (-D warnings, default + unstable), csharpier, and cargo doc --deny warnings.
• Reviewer: confirm the wasm bindings + C#/C++ test suites pass in CI (especially the C++ wasm build, which couldn't run locally).

[gefjon]

We might want to get someone else's eyes on the C# and C++ changes, but then again, we might not, 'cause these changes are pretty straightforward. From what I saw, those parts seemed reasonable. The Rust and docs changes are good. I'm ignoring llms.md, as it seems to be unrelated.