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807c9c7318
Closes the highest-priority remaining catalogue item (§G). User code queues PPU writes during `on frame` via three new intrinsics; the NMI drains the 256-byte ring at `$0400-$04FF` to `$2007` during vblank. Programs that never touch the buffer pay zero bytes and zero cycles for the feature — verified by the existing 46 ROMs all matching their goldens with no drift. Also fixes the failing CI Format check from7b4570eby running cargo fmt across the working tree. **Runtime:** - New `runtime::gen_vram_buf_drain` emits the drain routine (`__vram_buf_drain`). Walks entries `[len][addr_hi][addr_lo] [byte_0]...[byte_(len-1)]` and stops at `len == 0`. Uses `LDA $0400,X` indexed-absolute so no ZP scratch is needed. Drain costs ~12 setup cycles + 8 cycles per data byte; the 256-byte buffer can hold ~50 single-tile writes that drain in roughly 1000 cycles, well inside the ~2273-cycle vblank. - `NmiOptions` gains `has_vram_buf`. The NMI JSRs the drain after the existing palette/background handshake (compiler- queued PPU writes win priority for vblank cycles). **IR + codegen:** - Three new ops `IrOp::NtSet`, `IrOp::NtAttr`, `IrOp::NtFillH`. - The codegen helpers compute the PPU address inline: `$2000 + y*32 + x` for nametable, `$23C0 + (y/4)*8 + (x/4)` for attribute. Each append lays down a fresh `0` sentinel so the NMI sees a well-formed buffer regardless of whether more entries get appended later in the frame. - `__vram_buf_used` marker drops on first use; gates the runtime splice + NMI JSR. **Analyzer:** - AST-walking helper `program_uses_vram_buf` detects intrinsic use at analyze-init time so the user-RAM bump pointer can start at `$0500` (past the buffer) rather than the legacy `$0300`. Programs that don't use the buffer keep the legacy start. - Three intrinsic names registered in `is_intrinsic` / `is_void_intrinsic` with arity checks. **Tests + example:** - `examples/vram_buffer_demo.ne` exercises all three intrinsics on a backgrounded program — three single-tile score writes, a 16-tile horizontal fill, and an attribute write that flips the top-left metatile group's palette to red. Committed golden + audio hash. - Four new integration tests: byte-level JSR-to-drain assertion, drain-omitted-when-unused, RAM-bump assertion for programs that DO use the buffer, and arity enforcement for `nt_set`. **CI fix:** - `cargo fmt` ran across the tree. Picks up a one-line fmt diff in `tests/integration_test.rs` that the prior commit shipped without running fmt, causing the Format CI job to fail on `7b4570e`. All 758 tests pass. Clippy clean. 47/47 emulator goldens match.
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