fix(registers): avoid widened xHCI MMIO accesses

[ZR233]

Problem

The aggregate volatile accessors for the Port Register Set and the Supported Protocol header expose 16-byte MMIO values. With Rust nightly 2026-07-15 on AArch64, those accesses can be lowered to widened transactions. The RK3588 DWC3 controller on OrangePi-5-Plus then stalls during xHCI initialization.

The xHCI 1.2c Requirements Specification Table 5-19 (§§5.4.8–5.4.11) defines the four port registers as Dwords at offsets 0x0, 0x4, 0x8, and 0xc with a 0x10 port stride. Figure 7-1 and Tables 7-6–7-9 (§7.2) define the Supported Protocol header as four Dwords. HCCPARAMS1.AC64 (§5.3.6, Table 5-13) only says whether the high-order halves of 64-bit pointer/address fields are implemented; it does not change these 32-bit register sizes.

Changes

• replace the aggregate Port Register Set value with an owner that keeps the base address, port count, and mapper
• add lifetime-bound port(index) and port_mut(index) handlers whose four fields are independent 32-bit accessors
• add a read-only HeaderAccessor that reads four Dwords separately and reconstructs the existing Header value in ordinary memory
• add RecordingMapper regression tests for both paths
• document the port API migration and the widened-access fix in the changelog

The common Supported Protocol call remains unchanged:

let header = protocol.header.read_volatile();

Port callers migrate from aggregate reads and updates:

let portsc = registers.port_register_set.read_volatile_at(index).portsc;
registers.port_register_set.update_volatile_at(index, |port| {
    port.portsc.set_port_power();
});

to register-sized accessors:

let portsc = registers.port_register_set.port(index).portsc.read_volatile();
registers
    .port_register_set
    .port_mut(index)
    .portsc
    .update_volatile(|portsc| portsc.set_port_power());

Regression evidence

On the original implementation, both new tests failed with one 16-byte mapping:

actual:   [(base, 16)]
expected: [(base, 4), (base + 4, 4), (base + 8, 4), (base + 12, 4)]

Both tests pass after the change.

Validation

Passed locally:

• cargo +stable fmt --all -- --check
• cargo +stable test
• cargo +stable build
• cargo +stable clippy --all-targets
• cargo +nightly-2026-07-15 test
• cargo +nightly-2026-07-15 check --target aarch64-unknown-none-softfloat
• TGOSKits local-path consumer: cargo xtask clippy --package crab-usb
• TGOSKits local-path consumer: cargo test -p crab-usb (33 tests)
• OrangePi-5-Plus RK3588: cargo xtask starry test board --board orangepi-5-plus (all 8 board groups passed, including xHCI enumeration and USB2 lsusb)

The requested strict Clippy command with -D warnings -D clippy::pedantic -D clippy::all reports the same 39 pre-existing diagnostics on this branch and on upstream/main@f2254c86. RUSTDOCFLAGS="-D warnings" cargo +stable doc --no-deps is likewise blocked on the base commit by the renamed private_doc_tests lint in src/lib.rs. This change introduces no additional diagnostics in either check.

This references #158 without closing it because the mapping-performance part remains unresolved. The broader capability-access audit is tracked in #176.