Add Pi throttling / undervoltage state (vcgencmd get_throttled)

[LarsLaskowski]

Summary

Adds a new throttled object to the metrics snapshot that decodes the
Raspberry Pi vcgencmd get_throttled bitmask into individual booleans —
the single most-requested Pi health signal, since under-voltage and
thermal throttling silently degrade performance.

Each condition is exposed both as a current-state flag (*_now) and a
latched-since-boot flag (*_since_boot):

• under_voltage_now / under_voltage_since_boot
• frequency_capped_now / frequency_capped_since_boot
• throttled_now / throttled_since_boot
• soft_temp_limit_now / soft_temp_limit_since_boot
• raw — the original hex bitmask string (e.g. "0x50005")

Implementation notes:

• New internal/collector/throttled.go + throttled_test.go. The parser
  (parseThrottled) is unit-testable against fixture bitmask strings,
  independent of real hardware, per the acceptance criteria.
• vcgencmd is discovered lazily using the same throttled re-detection
  pattern (and shared detectRetryInterval) as the temperature collector,
  so a firmware tool appearing after startup is picked up without a restart.
• The change is additive: throttled is tagged omitempty and the
  whole object is omitted when vcgencmd is unavailable (e.g. off-Pi),
  so it degrades gracefully and does not alter existing v1 field shapes.

Related Issue

Closes #13

Checklist

• Tests added/updated for the change (go test ./... passes locally) —
  table test decodes fixture bitmasks (0x0, 0x50005, 0x50000,
  0xf000f, 0x80002) to the correct flag set, plus malformed-input
  and no-vcgencmd cases
• go vet ./... and golangci-lint run are clean (0 issues)
• Documentation updated (docs/API.md: added throttled to the
  /api/v1/metrics example and a note explaining the flags)
• No breaking change to /api/v1/... response shapes — the field is
  purely additive (omitempty)

Verification note

Verified via unit tests and off-Pi build/vet/lint. The live
vcgencmd get_throttled invocation path could not be exercised here
because this environment has no Raspberry Pi firmware tooling; on-hardware
confirmation that the collector reads and decodes a real bitmask is still
recommended.

[LarsLaskowski]

Review: Add Pi throttling / undervoltage state (vcgencmd get_throttled)

Reviewed against the PiMonitor Go/security/API conventions. This is a clean, well-scoped change — nice work. Approving in spirit; one non-blocking gofmt nit inline.

Verification (PR branch, off-Pi)

• go build ./... ✅
• go vet ./... ✅
• go test ./... -race -cover ✅ (collector package 86.9%)

Checklist

• Error handling ✅ — exec.CommandContext(...).Output() and parseThrottled errors are checked and wrapped; missing vcgencmd returns (nil, nil) and degrades gracefully off-Pi.
• Resource leaks ✅ — context.WithTimeout paired with defer cancel(); no open files/readers.
• /proc//sys (bitmask) parsing robustness ✅ — malformed / empty / missing-prefix / non-hex inputs are rejected and covered by TestParseThrottled_Malformed. ParseUint(raw, 0, 64) correctly infers the 0x base.
• Command execution safety ✅ — fixed argument list (c.vcgencmdPath, "get_throttled"), no shell string-concatenation.
• REST API stability ✅ — additive *Throttled field with omitempty; existing v1 shapes unchanged, docs/API.md updated with example + note.
• Test coverage ✅ — table test over five real bitmasks, malformed cases, and the no-vcgencmd path. (The live Collect → vcgencmd invocation can't be exercised off-Pi, as the PR notes; on-hardware confirmation still recommended.)
• Language ✅ — all English.
• Dependencies ✅ — none added; lazy detection reuses the shared detectRetryInterval and mirrors TemperatureCollector.

One nit (non-blocking)

• internal/collector/collector.go isn't gofmt-clean — adding the longer throttled field widened the alignment column but the surrounding fields (and the New(...) literal) weren't re-aligned. CI won't flag it (no gofmt linter configured), so it would merge as-is. gofmt -w internal/collector/collector.go resolves it. See inline comment.

Everything else looks good to merge once that's tidied up.