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/state_portal.rs

@@ -1,4 +1,7 @@
 // 采集门户状态模块 —— 通过 PipeWire/DMA-BUF 进行屏幕采集并编码
+// AsRawFd is required by frame.fd.as_raw_fd() in build_drm_descriptor below
+// but rustc emits a false "unused_imports" warning because OwnedFd also has
+// an inherent as_raw_fd — same quirk as avhw.rs. E0599 if removed → keep it.
 use std::os::fd::AsRawFd;
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicBool, Ordering};