esp32: fix linker layout, UART/I2C clocking, and print interleaving - #5617

Found by cross-checking the esp32-generic target against the real
esp-idf sources, after confirming the WiFi/BT blob crashes traced
back here rather than to the blob itself.

- targets/esp32.ld: IROM region was sized at a flat 4M instead of the
  real SOC_IROM_HIGH - SOC_IROM_LOW (0x330000) limit, and the .iram
  section pattern list was missing *(.coexsleepiram*), a WiFi/BT-blob
  section confirmed present via objdump on libcoexist.a. An unmapped
  section here means the blob's power-management ISR runs out of
  whatever memory happened to follow, which shows up as sporadic
  crashes during radio activity.
- src/machine/machine_esp32.go: UART never set
  CONF0_TICK_REF_ALWAYS_ON, which only worked by luck on UART0 because
  the ROM bootloader happens to leave it in APB clock mode; any other
  UART or a cold path through this code gets the wrong clock source.
  Also fixes the baud divider to keep its fractional part instead of
  truncating to a bare integer division.
- src/machine/machine_esp32_i2c.go: SCL_HIGH_PERIOD compensation was
  scaled by clock frequency (halfCycle/8) instead of using the fixed
  offset esp-idf's noise-filter formula actually calls for
  (thres+6, thres=7 → 13), producing wrong SCL timing at anything
  other than the frequency it happened to be tested at.
- src/runtime/scheduler_cooperative.go: printlock/printunlock were
  no-ops, so two goroutines calling println concurrently can
  interleave their output mid-line on this cooperative scheduler
  (print can yield, e.g. waiting on a UART FIFO, before it finishes
  writing). Added a real bounded-wait lock.

go.mod/go.sum: point tinygo.org/x/espflasher at flrossetto/espflasher
(fix-esp32-chip-detect-and-stub-baud-cap) until that fix lands
upstream — see that repo for details on the chip-detection and flash
baud-rate fixes needed for reliable esp32-generic flashing.