S14.08: Make SolidSyslogFormatter private to the library

[DavidCozens]

Parent epic: #64

Context

With SD authoring (S14.06) and header fields (S14.07) both off the public
formatter, SolidSyslogFormatter has no remaining public consumer. Move it out of
the public interface entirely — the structural payoff of the epic — and write the
design note that records the new SD-authoring API.

What changes

• Move SolidSyslogFormatter.h from Core/Interface/ to Core/Source/.
• Remove the now-unnecessary struct SolidSyslogFormatter; forward declarations
  from any public headers / callback typedefs.
• Update the CLAUDE.md public-header table: remove the SolidSyslogFormatter
  row; add SolidSyslogSdElement / SolidSyslogSdValue / the header-field-writer
  rows.
• Author docs/structured-data.md — the design note (two-type writer,
  escaping, streaming + Option-B continuation state, SD-NAME validation, custom
  SD-ID), which becomes the skeleton for the S14.09 integrator guide.

Acceptance criteria

• No file under Core/Interface/ or Platform/*/Interface/ references
  SolidSyslogFormatter.
• Library + tests build with the formatter private (formatter primitives still used
  freely inside Core/Source — timestamp formatter, block-device filenames, the SD
  / token writers).
• CLAUDE.md public-header table updated.
• docs/structured-data.md exists and describes the agreed design.
• DEVLOG + memory updated per cadence.

Out of scope

• The integrator how-to / worked example / oracle BDD (S14.09).

Areas affected

Core/Interface/, Core/Source/, CLAUDE.md, docs/structured-data.md.
Largely a mechanical move plus documentation.

[DavidCozens]

Scope note for the PR (#558): the SD-authoring design note docs/structured-data.md listed in this issue is deferred to S14.09, where it lands beside the integrator guide rather than alone here — it's the natural neighbour, and the two would otherwise overlap. #558 delivers the rest of #548: the SolidSyslogFormatter move to Core/Source/ (private), the new SolidSyslogEndpointHost writer that unblocked it (the endpoint host was the last public formatter consumer, which this issue overlooked), and the CLAUDE.md public-header-table revision. E14 closes with S14.09.