feat(fpga): DSLogic U2basic integration

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Add DSLogic U2basic logic analyzer support:

• src/cli/fpga_dslogic.zig - CLI tool with 8 commands
• fpga/DSLOGIC_U2BASIC_CONNECTION.md - Bilingual connection guide
• fpga/dslogic_channels.json - Channel-to-ball mapping
• fpga/dslogic_presets.json - DSView presets
• Updated build.zig to include fpga_dslogic module
• Updated .trinity/fpga/hardware_state.json with DSLogic section
• Updated .trinity/fpga/experience.json with DSLOGIC-001

Phase 1-4 complete (documentation, configuration, CLI integration). Phase 5 (pin mapping verification) requires hardware.

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Partially Complete — Zig 0.15 API compatibility fixes

Phases 1–3 (Railway GraphQL, db/sessions, db/client, main.zig, config.zig, server.zig) — all committed.
Remaining Zig 0.15 errors (HTTP SplitIterator, ArrayList) — moved to #505 for proper migration.