host: volume option; docs: voice adjustments beyond speed (volume, Flextalk profile)

docs/TTS-DESIGN.md

@@ -260,10 +260,36 @@ record what the engine needs to interpret, add, or hide._
 ### Text handling
 
 - SAPI4 is ANSI/CP1252. The wrapper converts UTF-8 to CP1252 and maps emoji and
-  unmappable characters to spaces. Engine tag parsing is disabled, so input stays plain
-  text (no SSML), matching the contract.
+  unmappable characters to spaces. The host passes `TTSDATAFLAG_TAGGED`, so SAPI4 control
+  tags in the text are enabled at the engine level; tagged input is not part of this
+  contract and is untested. Plain text (no SSML) is the contract.
 - Lines up to 1000 characters are well within engine limits.
 
+### Other voice adjustments (beyond speed)
+
+All voices implement SAPI4 `ITTSAttributes`, so **pitch, speed and volume** exist at the
+API level. The Veskslop contract exposes only speed and pitch; volume is available in the
+host as `sapi4host -V <value>` and is not sent by Veskslop.
+
+- **Volume** (measured 2026-09-15):
+  - Flextalk: linear, range 0-65535, default 65535; half (32767) is about -6 dB, 0 is
+    silence.
+  - MSTTS: default is 0xFFFFFFFF; 0 is silence, but the response is not linear (for
+    example 4096 gives about -24 dB while 65535 and larger are full scale). Treat volume
+    as effectively on/off for MSTTS.
+- **Flextalk per-speaker profile** (applied at voice load, not runtime-adjustable):
+  values live in `HKLM\Software\AT&T\Watson\2.0\FlexTalk\2.0\Speakers\{GUID}` (see
+  `docs/NOTES.md` for the full list and defaults). `Pitch`/`Speaking Rate` set the natural
+  voice; **Expression** was verified to change the synthesis audibly (Keith Bell at
+  Expression 0 / 59 / 100 yields different waveforms with the same duration). Changing a
+  profile requires a registry edit and re-selecting the voice.
+- **MSTTS voice character** is fixed by the `.vce`/`.cfg` data; the engine exposes no
+  per-voice profile in the registry. Its effect variants (telephone, in Hall, in Stadium,
+  in Space) are separate voice modes with separate data, exposed as distinct voice IDs
+  (`mike-telephone`, `mary-hall`, ...), not runtime effects.
+- Not exposed by this integration: SSML, prosody/style tags, or SAPI4 attribute
+  extensions beyond pitch/speed/volume.
+
 ### Transport and latency
 
 - Installation is one command: `setup/install.sh` (system dependencies with sudo, wine