feat(fpga): SPI flash program/dump and boot-from-flash mode-pin resolution #1288

Add tri fpga program-flash and tri fpga dump-flash subcommands wrapping openFPGALoader'''s JTAG-to-SPI bridge for the XC7A200T-FGG676 board. Flash write + verify succeed; boot-from-flash is blocked by the board mode-pin strapping and needs investigation.

Acceptance:

• tri fpga program-flash writes the GF16 bitstream to SPI flash and verifies.
• tri fpga dump-flash reads back the flash contents.
• Board boots from flash after power cycle (or mode-pin limitation is documented).
• Conformance suite stays at 575/575 PASS.

User Scenarios

• Given a GF16 bitstream file and an XC7A200T-FGG676 board connected via JTAG, when the user runs tri fpga program-flash <bitstream>, then the bitstream is written to SPI flash and verified.
• Given an XC7A200T-FGG676 board with SPI flash programmed, when the user runs tri fpga dump-flash <output-file>, then the flash contents are read back to the output file.
• Given the board is power-cycled after flash programming, when the board boots, then it loads the bitstream from SPI flash (or the mode-pin limitation is documented).

Requirements

FR-001: The tri fpga program-flash subcommand MUST write the GF16 bitstream to SPI flash using openFPGALoader's JTAG-to-SPI bridge and verify the write.
FR-002: The tri fpga dump-flash subcommand MUST read back the SPI flash contents using openFPGALoader's JTAG-to-SPI bridge.
FR-003: The implementation MUST investigate and document the board mode-pin strapping limitation that blocks boot-from-flash.
FR-004: The conformance suite MUST remain at 575/575 PASS after the changes.

Success Criteria

• tri fpga program-flash <bitstream> writes and verifies the GF16 bitstream to SPI flash, exit 0
• tri fpga dump-flash <output-file> reads back flash contents to output file, exit 0
• Board boot-from-flash behavior documented (works or mode-pin limitation documented)
• Conformance suite passes 575/575 tests

Boundary

• specs/tools/tri/fpga.t27
• cli/tri/src/fpga.rs
• scripts/fpga/flash.sh
• specs/port/scripts/fpga/flash.t27