bscan: rename from bscan_spi, add TAP primitives for the SVF player

The module outgrew its "SPI bridge" name — it's mostly generic
single-device JTAG TAP primitives now. Rename modules/bscan_spi ->
modules/bscan (dir, files, library target, includes, doc paths);
bscan_* function names and bscan_spi_xfer() kept.

Add the two primitives the SVF player needs beyond shift_dr:
- bscan_shift_ir: general IR scan with TDO capture (bscan_set_ir is
  opcode-only, no readback)
- bscan_tap_reset: force Test-Logic-Reset, land in Run-Test/Idle

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -156,7 +156,7 @@ modules/
 ├── bus_over_jtag/   SPI / I²C / MDIO / parallel mem bit-bang (EXTEST)
 ├── drivers/         FTDI, J-Link, Linux GPIO, LPT, Digilent (optional)
 ├── fpga/            Registry loader (parses fpga_registry.yaml at runtime)
-├── bscan_spi/       BSCAN proxy loader + fast SPI-over-USER1 bridge
+├── bscan/           JTAG TAP primitives + BSCAN proxy (bitstream, SPI-over-USER1)
 ├── spi_flash/       SPI NOR chip database + read/erase/program/verify
 ├── script/          Script engine
 ├── config/          Built-in config.script