docs: what / why proposal for stream token counting via SSE eent parsing

[szedan-rh]

issue: #211
discussion: #20
status: proposed
authors:

• szedan-rh
  graduation_criteria:
• SSE parser correctly accumulates token counts for all five providers
• Both delta and cumulative provider semantics handled
• Stream completion detection works for all sentinel formats
• Token counts written to FilterContext via set_token_usage()
• Non-streaming responses parsed in the same filter
• Unit tests cover all provider streaming formats
  stakeholders:
• shaneutt

[praxis-bot]

PR Review

Summary: Adds a what/why proposal document for streaming token counting via SSE event parsing (issue #211).

Overall: The proposal is well-structured with clear motivation, good provider comparison tables, and appropriate scope boundaries. Three issues need attention: all 12 markdown reference links are broken (never defined), the "User Stories" section required by the proposal template is missing, and the frontmatter graduation_criteria field defines implementation-level criteria that are premature for a what/why-only proposal.

Severity	Count
Critical	0
Large	1
Medium	2

Reviewed by praxis-bot

[praxis-bot]

[Large] All 12 reference-style markdown links in this document ([#216], [#510], [#212], [#474], [#491], [#493], [#219], [#214], [#21], [#239], [#97], [#20]) are never defined. They render as literal bracketed text, not clickable links. Add link definitions at the bottom of the file, e.g.:

[#20]: https://github.com/praxis-proxy/praxis/issues/20
[#21]: https://github.com/praxis-proxy/praxis/issues/21
[#97]: https://github.com/praxis-proxy/praxis/issues/97
[#212]: https://github.com/praxis-proxy/praxis/issues/212
[#214]: https://github.com/praxis-proxy/praxis/issues/214
[#216]: https://github.com/praxis-proxy/praxis/issues/216
[#219]: https://github.com/praxis-proxy/praxis/issues/219
[#239]: https://github.com/praxis-proxy/praxis/issues/239
[#474]: https://github.com/praxis-proxy/praxis/pull/474
[#491]: https://github.com/praxis-proxy/praxis/pull/491
[#493]: https://github.com/praxis-proxy/praxis/pull/493
[#510]: https://github.com/praxis-proxy/praxis/pull/510

See 00216_provider-token-mapping.md for the existing convention.