gpio: share line requests across read/write instances on the same pin

[andig]

fixes #31506

PR #27815 replaced go-rpio (direct register mmap, no exclusive line ownership) with go-gpiocdev (Linux GPIO character-device uAPI, exclusive per-line requests). Any config using the same GPIO pin for both a read function and a write function - as switchsocket configs commonly do, reading back a relay's own driven state on the same pin used to switch it - now fails at startup with "failed to open GPIO: device or resource busy", since the second RequestLine call for that pin is rejected by the kernel.

• gpio plugin instances now request lines through a shared per-(chip,pin) registry instead of each calling gpiocdev.RequestLine independently
• a line already open for input is reconfigured to output on demand (an output line's driven value can still be read back), so whichever instance needs write access "wins" the direction while read instances keep working off the same line
• this restores behavior equivalent to pre-Gpio plugin: Raspberry Pi 5 compatibility #27815, without reverting the Raspberry Pi 5 compatibility fix

Not unit-tested: this file is //go:build linux-only and talks to real /dev/gpiochipN hardware, consistent with it having no existing tests before this change. Verified via GOOS=linux GOARCH=arm(64) go build/vet and golangci-lint.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="plugin/gpio_linux.go" line_range="40-41" />
<code_context>
+
+	key := chip + ":" + strconv.Itoa(pin)
+
+	if sl, ok := lines[key]; ok {
+		if output && !sl.isOutput {
+			if err := sl.line.Reconfigure(gpiocdev.AsOutput(0)); err != nil {
+				return nil, fmt.Errorf("failed to reconfigure GPIO: %w", err)
</code_context>
<issue_to_address>
**issue (bug_risk):** Line reconfiguration should be synchronized with sharedLine.mu to avoid races with concurrent reads/writes.

In `acquireLine`, reconfiguration of an existing line only holds `linesMu` while calling `sl.line.Reconfigure` and updating `sl.isOutput`, whereas getters/setters use `sl.mu`. This allows `Value`/`SetValue` to run concurrently with reconfiguration, risking races at the driver/kernel level. Please also hold `sl.mu` when reconfiguring (e.g., lock `sl.mu` before `Reconfigure` and updating `isOutput`) so all accesses to `sl.line` use the same mutex.
</issue_to_address>

---

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[andig]

/build

[github-actions]

✅ Build finished.

• Binaries: download artifacts
• Docker: docker pull evcc/evcc:pr-31508