chore: fail-fast CI hangs + refresh README header

[Sunrisepeak]

Summary

Recent main runs (notably the v0.0.28 bump) burned the full 60-min job budget on 10_env_command.sh hanging in xlings install on a fresh MCPP_HOME. The hang is intermittent and lives at the network boundary; no fix to xlings itself here — just shorten the feedback loop so the offending test is identified in 10 min instead of 60.

The PR also brings the README header up to v0.0.28 reality (Windows LLVM is now ✅, CI is green on all three platforms) and elevates the key external links above the fold so newcomers don't have to scroll to find docs / mcpp-index / mcpplibs / forum.

CI changes

• tests/e2e/run_all.sh — wraps every test with timeout 600 (override via E2E_TEST_TIMEOUT). Distinguishes TIMEOUT (exit 124) from regular FAIL and surfaces the offending test name in the summary. Uses timeout if available, falls back to gtimeout (macOS coreutils), else skips wrapping and relies on the step-level guard.
• ci.yml / ci-macos.yml / ci-windows.yml — add timeout-minutes: 25 to each E2E suite step. A hung suite now fails in ~25 min instead of eating the full 60-min job, freeing the downstream toolchain test steps to still run / be diagnosed.

Behaviour matrix

Scenario	Before	After
10_env_command.sh stalls in xlings download	60 min wall-clock → step cancelled with no clue which test	Single test killed at 10 min → TIMEOUT: 10_env_command.sh (exceeded 600s — likely network / xlings stall) printed; rest of suite continues; step cap at 25 min
All tests pass	unchanged	unchanged (just adds Per-test timeout: 600s (via timeout) line at start)
Real test failure	unchanged	unchanged (FAIL: NN_x.sh (exit N))

Local verification

$ E2E_TEST_TIMEOUT=3 bash run_all.sh
...
=== 99_hang.sh ===
synthetic hang test starting
TIMEOUT: 99_hang.sh (exceeded 3s — likely network / xlings stall)

E2E Summary: 1 passed, 1 failed, 0 skipped
Timed out: 99_hang.sh
Failed: 99_hang.sh (TIMEOUT)
$ echo $?
1

README changes

• New centered links table directly under the badges (2 rows):
  • row 1: docs / quick start / mcpp.toml guide / examples / toolchains
  • row 2: mcpp-index / mcpplibs / forum / Issues / Releases
• Add live CI status badges for Linux / macOS / Windows; drop the static Self-hosted badge that the CI badges now subsume.
• Promote Windows x86_64 Clang/LLVM from 🔄 to ✅ (CI green since v0.0.27); add footnote noting the MSVC BuildTools dependency and the future llvm-mingw direction.

Test plan

• ci.yml green (Linux self-host, E2E suite under 25 min)
• ci-macos.yml green
• ci-windows.yml green (incl. the stdin regression step from fix: seal child-process stdin on Windows (first-run hang) #74)
• If 10_env_command.sh hangs in CI again, the failure log shows TIMEOUT: 10_env_command.sh instead of Error: The operation was canceled.