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refactor: propagate const through internal transmit chain (phase 1 of 2) (#228)

* refactor: propagate const through internal transmit chain

Make internal transmit functions accept const uint8_t * buffers,
reflecting that the transmit path never modifies its input data.

This resolves scattered const-discarding casts at call sites (e.g.
(uint8_t *)c_newline, (uint8_t *)request) by giving them a
const-correct destination type. The internal nTransmitFn typedef
is updated to match.

At the two public API boundaries (_noteSerialTransmit and
_noteI2CTransmit), an intentional cast back to uint8_t * is added
with an explanatory comment, consolidating the remaining const
inconsistency to a single known location per interface. These casts
will be removed in a follow-up breaking change when serialTransmitFn
and i2cTransmitFn are updated to accept const uint8_t *.

No functional or behavioral change — purely a const-correctness
cleanup. The public hook typedefs (serialTransmitFn, i2cTransmitFn)
are intentionally left unchanged for this commit.

* fix: use stack buffers for flash-resident literals in transmit calls

Replace direct casts of flash-resident string literals with small
stack-allocated buffers at the three call sites where .rodata data
was passed through the non-const transmit hook boundary:

- n_serial.c: c_newline (\r\n) appended after JSON body
- n_serial.c: "\n" sent during serial reset/resync
- n_i2c.c:    "\n" sent during I2C reset/resync

Each buffer is 1-2 bytes. The stack cost is negligible, but the
consequence is meaningful: the hook receives a pointer to mutable
RAM rather than flash, eliminating the UB that would occur if any
hook implementation writes to the buffer before transmitting.

The intentional boundary casts in _noteSerialTransmit and
_noteI2CTransmit remain in place, but with these stack buffers the
pointed-to memory is now genuinely mutable, so no UB can arise even
if a hook writes to it.

TODO comments on each buffer mark them... (continued)

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