fix: read /etc/os-release with one parser that matches sh

[rominf]

Fixes #543.

/etc/os-release was parsed in four places, each with its own trim_matches, and against sh sourcing the same file they disagreed with each other:

file	sh	driver plan	rocm examine	OpenMPI hint
ID='ubuntu'	ubuntu	'ubuntu'	'ubuntu'	ubuntu
ID=debian then ID=ubuntu	ubuntu	debian	ubuntu	debian

So a spec-valid single-quoted file read as an unsupported distro, and on a file with a duplicated ID, rocm examine reported Ubuntu while the driver plan was built for Debian 12.

The fix

All four readers now go through rocm_core::os_release, which keeps one rule: the value sh would assign, or nothing.

• For a file of blank lines, comments and spec-valid assignments it returns sh's value: one matching quote pair removed, \" \\ \` \$ decoded inside double quotes, nothing decoded inside single quotes, \x meaning x in a bare value, a # comment after a value, the last assignment winning.
• Any other line makes the whole file unreadable. To sh such a line is a command or the start of one (export ID=…, X=1; ID=…, unset ID, an unterminated quote that swallows the lines after it), and a command can change any variable — so reading past it would report values sh does not assign. CRLF files are unreadable too: sh assigns ubuntu\r, which matches no distro.
• An unreadable file is reported, not hidden. The driver plan says /etc/os-release line N is not a plain assignment: "<line>"; no driver commands were planned because the distro cannot be read. instead of "this distro is not supported", and rocm examine records the same line in probe_failures.

Real files

Checked against dash: all 50 container images, and 396 of the 397 files in which-distro/os-release at 10dfd5bc, read exactly as dash reads them. The one exception is a CRLF file, which reads as unreadable. No file read a value dash does not assign.

Behaviour changes

• rocm examine reports single-quoted and duplicated keys as sh reads them, and an unreadable file as a probe failure naming its line.
• The driver plan — and the package installs it approves in ensure_openmpi_for_vllm and ensure_torch_runtime_dep — read the same distro rocm examine reports.
• The OpenMPI hint no longer accepts ID =x; to sh that runs a command named ID.

Tests

• The parser against /bin/sh itself, with an empty environment, a sentinel HOME and a PATH that does not exist: every spec-valid encoding of every value up to three characters, and every right-hand side up to three characters over a quote-, backslash- and newline-heavy alphabet, checked for the key it sets and a key after it. Each must read as sh reads it or not at all; a list of well-formed cases must read as a value; no earlier value may come back.
• Nine mutations of the parser's rules each turn the suite red.
• End to end through the driver plan: every supported distro rewritten in single quotes plans exactly as in double quotes; a duplicated ID plans from its last assignment; an unreadable file is refused with the message above and no commands. rocm examine's probe failure is asserted together with the state it describes.

Scenarios

There is no e2e scenario in this PR. /etc/os-release is read from its real path, and the seam that lets a scenario plant one is the host-root re-rooting in #510. Scenarios for a single-quoted file and a duplicated ID, through rocm install driver --dkms --dry-run, are written against that seam and follow once it lands.

Verification

On this commit: cargo fmt --check, both CI clippy -D warnings invocations, cargo test -p rocm-core --lib (474 passed), -p rocm --bin rocm (974 passed), -p xtask (255 passed), xtask manifest --check, tpn --check, check-crate-edges, and cargo xtask e2e (153 scenarios; the only failures are the two already marked expected). The new end-to-end tests were run against the code before this change and fail there for the reasons described above.

Locally, cli_progress::tests::animated_spinner_keeps_ticking_without_progress_calls — which needs three 5 ms ticks inside 60 ms — failed in two full runs on a heavily loaded machine and passed five of five on its own; it is unchanged by this PR. Not run locally: the Windows lane.