E21: Port-Time Configurability

[DavidCozens]

Overview

Some library-internal constants are deliberately tuned for a default
hosted/embedded balance, but real users on memory-constrained or
latency-sensitive targets will want to tune them at build time. This
epic gathers those candidates under a single CMake override
mechanism, plus the documentation needed for a porter to actually
use it.

Approach

The shape, already in place from S21.01 (#347):

-D<NAME>=<value>  ->  #define <NAME> <value>  ->  enum default guarded by #ifndef

CMake exposes cache variables that surface as preprocessor defines.
Each tunable lives as an enum default in code, guarded by #ifndef
so a -D override wins. Per-target overrides flow through
SOLIDSYSLOG_USER_TUNABLES_FILE (a single integrator-owned header).

Done

• SOLIDSYSLOG_MAX_MESSAGE_SIZE — S21.01 (S21.01: SOLIDSYSLOG_MAX_MESSAGE_SIZE as the first build-time tunable — mbedTLS-style override mechanism #347, mechanism + first
  tunable, default unchanged at 2048) and S21.02 (S21.02: Tune SOLIDSYSLOG_MAX_MESSAGE_SIZE to 512 on the freertos-cross preset #349, retuned to 512
  on the freertos-cross preset).
• SOLIDSYSLOG_MAX_PATH_SIZE and SOLIDSYSLOG_MAX_INTEGRITY_SIZE
  — S21.03 (S21.03: Lift MAX_PATH_SIZE and MAX_INTEGRITY_SIZE to tunables #446) lifted both into SolidSyslogTunablesDefaults.h in
  one bundled story. Defaults unchanged (128 and 32); floors 32 and 4
  respectively.

Absorbed by other epics

• TCP connect + TLS handshake timeouts — originally listed here
  under SEND_TIMEOUT_* (since renamed to CONNECT_TIMEOUT_* and
  HANDSHAKE_TIMEOUT_* during S12.14/S12.16). Delivered by
  S12.17 (S12.17: Tunable + runtime-overridable TCP connect and TLS handshake timeouts #282, merged PR feat: S12.17 tunable + runtime-overridable TCP connect and TLS handshake timeouts #447) under E12, which uses the E21
  tunables mechanism for the compile-time defaults
  (SOLIDSYSLOG_TCP_CONNECT_TIMEOUT_MS,
  SOLIDSYSLOG_TLS_HANDSHAKE_TIMEOUT_MS) and overlays a per-instance
  runtime getter + Null Object on top so integrators can also
  live-tune without rebuilding firmware.

Story — Store block-size architecture — S21.04 (#527)

The SolidSyslogBlockDevice vtable gains a GetBlockSize operation so
the device becomes the single source of truth, MaxBlockSize drops out
of SolidSyslogBlockStoreConfig, and all file-backed devices share one
SOLIDSYSLOG_FILE_BLOCK_SIZE tunable. API-breaking — scheduled into
the 0.x beta window so it lands before 1.0.0 (additive-only). Full
shape, blast radius, and acceptance criteria live in the story (#527).

Explicitly rejected

• MAX_BLOCK_INDEX = 99 in SolidSyslogFileBlockDevice.c. Coupled
  to the two-digit on-disk sequence in filenames and to
  BlockSequence::SEQUENCE_MODULUS = 100 for wrap detection. The
  100-block cap is high enough for any realistic use case and the
  value consumes no runtime memory. Leave hardcoded.

Documentation — folded into E23 (#246)

The porting / tunables documentation page (full list of overridable
constants, recommended ranges, -D / preset / SOLIDSYSLOG_USER_TUNABLES_FILE
usage, memory and test/BDD implications) is tracked under E23
(Documentation site), not as a separate E21 story — it belongs with
the rest of the porting docs rather than duplicating the docs workflow
here. The current per-tunable header comments in
Core/Interface/SolidSyslogTunablesDefaults.h remain the interim docs
(per S21.01's "the defaults header is the docs" decision) until E23
collects them.

Stories

Story	Title	Status
S21.01 (#347)	SOLIDSYSLOG_MAX_MESSAGE_SIZE as the first tunable — mechanism	Done
S21.02 (#349)	Tune SOLIDSYSLOG_MAX_MESSAGE_SIZE to 512 on freertos-cross	Done
S21.03 (#446)	Lift MAX_PATH_SIZE and MAX_INTEGRITY_SIZE to tunables	Done
—	TCP connect + TLS handshake timeout tunables (compile-time side)	Absorbed by S12.17 (#282)
S21.04 (#527)	BlockDevice reports block size + single SOLIDSYSLOG_FILE_BLOCK_SIZE tunable	Todo
—	Porting / tunables documentation page	Moved to E23 (#246)

[DavidCozens]

All four stories complete (S21.01–S21.04, #527 merged via #528). Block size is now a property of the device with a documented grow-and-warn floor; the porting/tunables documentation page was folded into E23. Closing E21.