feat(runtime): implement debugger STRING / WSTRING read/write/force/unforce

[thiagoralves]

Summary

Replace the four *_string_stub placeholders in debug_dispatch.hpp with real implementations against IECStringVar<254> / IECWStringVar<254>. Net effect for the editor: STRING / WSTRING variables in the debug watch panel will show their actual values instead of -.

Root cause

The stubs were a deliberate Phase-4a placeholder. The dispatcher's handle_read short-circuits with zero bytes when type_ops[].size == 0, and handle_set / handle_write reject any payload with STATUS_DATA_TOO_LARGE. Both STRING and WSTRING rows had size = 0.

Wire format (already expected by the editor decoder)

Matches the len8-utf8 / len8-utf16le arms in openplc-web/src/frontend/utils/variable-sizes.ts::decodeWireValue:

Type	Width	Layout
STRING	127 B	[ uint8 length ][ 126 bytes UTF-8 ]
WSTRING	253 B	[ uint8 length ][ 126 char16_t LE code units ]

WSTRING serialises char16_t as explicit little-endian byte pairs so the wire format stays host-endianness-independent.

Force semantics

• read uses the wrapper's .length() / .c_str() proxies (added in v0.5.1) which route through forced_value_ when forcing is active — so a debugger watch shows the forced value, not the underlying program-side value.
• write uses IECStringVar::set, which is itself a no-op when the variable is forced. Matches OPC-UA / debug-soft-write semantics: soft writes don't override a force.
• force pins via IECStringVar::force; unforce clears via IECStringVar::unforce.

Why widths matter

Setting size = 127 / 253 means handle_read returns the full window every poll. The editor's decoder treats the leading length byte as authoritative and only renders min(length, 126) characters; the runtime zero-fills the trailing window so stale bus contents from a previous read can't leak into the displayed value.

Verified end-to-end

Direct C++ smoke test (compiled with clang++ against the patched header):

STRING read 'hello'  → len=5 bytes='hello'                       ✓
WSTRING read 'hi'    → len=2 bytes=68 00 69 00 (UTF-16LE 'h','i') ✓
STRING write 'world' → IECStringVar.length()=5, c_str()='world'   ✓
STRING force 'forced'→ is_forced=1, c_str()='forced'              ✓
STRING unforce       → is_forced=0, c_str()='world' (back)        ✓

Test plan

• Direct C++ smoke test passes for all 4 ops on STRING + WSTRING.
• Full local CI-equivalent: lint (0 errors), prettier clean, typecheck clean, 1913/1920 tests pass (7 pre-existing skips).
• CI green on this PR.
• After v0.5.3 release: editor + web pick up the new strucpp, debugger watch panel shows STRING / WSTRING values for the Irrigation Controller dev project's MANUAL_OVERRIDE0.test_string.

Downstream

This unblocks the debugger STRING display in openplc-editor / openplc-web. After this lands and v0.5.3 is tagged, the version bumps in both repos' binary-versions.json propagate the fix.

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