chore: bump MSRV to 1.87 to match actual API usage

Oracle P2 follow-up. The clippy --fix autofixes earlier in this branch
silently introduced dependencies on APIs newer than the README's
1.70+ claim:
  - u32::is_multiple_of  (stable 1.87)
  - Option::is_none_or   (stable 1.82)

clippy::incompatible_msrv flagged the mismatch once rust-version was
pinned. Bumping the floor to 1.87 is the honest fix — the codebase
genuinely depends on 1.87 features now, and 1.87 has been stable
long enough (current stable is 1.96) that desktop CLI users on stable
Rust already have it.

  - Cargo.toml: rust-version '1.70' -> '1.87'. Comment lists the specific
    APIs that drove the bump and notes that further bumps need to be
    validated against clippy::incompatible_msrv.
  - README.md: Prerequisites line updated to 1.87+ with a brief why.
  - src/state_portal.rs: added the AsRawFd rustc-quirk comment that was
    already in avhw.rs (rustc emits a false 'unused_imports' warning;
    removing it produces E0599). Same known quirk, same documentation
    pattern.
  - src/transform.rs: fixed empty_line_after_doc_comments warning by
    converting the leading // doc-style comment to a //! module-level
    doc comment (which is what it should have been when I rewrote the
    file in commit 145b5d3).

All 79 unit tests + 3 integration tests pass. clippy: 0 errors,
0 incompatible_msrv warnings, 0 empty_line_after_doc_comments warnings.
Remaining warnings are: 1 AsRawFd rustc false-positive (documented),
5 unnecessary_cast FFI false-positives (rustc quirk on pointer casts),
and 8 dead-code items that need product decisions.

src/transform.rs

@@ -1,11 +1,11 @@
-/// Coordinate transformation module for Wayland output transforms.
-//
-// Historically exposed a family of `Rect`/`screen_to_frame`/`fit_inside_bounds`
-// helpers for ROI-based capture clipping. Those were never wired into the
-// capture pipeline (we capture full frames and let FFmpeg's filter graph handle
-// any scaling/rotation); they have been removed. Only `Transform` and the
-// `transpose_if_transform_transposed` helper remain — both are actively used by
-// `state.rs` and `avhw.rs`.
+//! Coordinate transformation module for Wayland output transforms.
+//!
+//! Historically exposed a family of `Rect`/`screen_to_frame`/`fit_inside_bounds`
+//! helpers for ROI-based capture clipping. Those were never wired into the
+//! capture pipeline (we capture full frames and let FFmpeg's filter graph handle
+//! any scaling/rotation); they have been removed. Only `Transform` and the
+//! `transpose_if_transform_transposed` helper remain — both are actively used by
+//! `state.rs` and `avhw.rs`.
 
 /// Wayland output transform enum, matching `wl_output::Transform`.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]