refactor(core): split jcode-app-core into jcode-base + jcode-app-core (Phase B)

Split the 191K-loc application core into two separately-compiled rustc
units along its internal library dependency DAG:

- jcode-base (foundational layer, ~57K nontest loc / 58 modules): provider,
  auth, config, session, message, memory, telemetry, prompt, storage, and
  their supporting leaves. This is the downward-closed set the upper layer
  depends on (zero non-test edges back up to app-core).
- jcode-app-core (upper layer, ~77K nontest loc): the server/tool/agent SCC
  plus presentation-adjacent leaves (ambient, notifications, channel,
  external_auth, replay, setup_hints, update, ...). It re-exports jcode-base
  via , so every existing  path keeps
  resolving across both crates and the root jcode facade.

Measured peak rustc VmHWM (selfdev):
  baseline monolith        3.18  GiB
  Phase A app-core         2.643 GiB
  Phase B jcode-base       1.505 GiB
  Phase B jcode-app-core   1.621 GiB
  root jcode (cli+tui)     1.407 GiB
Largest compilation unit: 3.18 -> 1.62 GiB (-49%), and the three big units
are now balanced (1.41-1.62 GiB), well under the 15 GiB/no-swap OOM threshold
even with parallel jobs.

Mechanics:
- New crate crates/jcode-base (deps cloned from app-core; owns the jemalloc +
  embeddings features since process_memory/embedding moved here; pdf stays in
  app-core with tool/read.rs). Dev-dependency on jcode-app-core for the single
  cross-layer test (provider/gemini_tests.rs builds a tool::Registry); Cargo
  permits dev-dependency cycles and they do not affect the lib build.
- app-core forwards jemalloc/embeddings features to jcode-base; keeps pdf.
- git mv of 58 base modules (+ their #[path]-included *_tests.rs files,
  memory_prompt.rs, session_tests/) preserving history.
- Promote the cross-boundary provider state cluster (ProviderRuntimeState,
  ProviderStateEvent, ProviderModelSelectionSource + observed/apply/
  selected_model/selection_generation/user_selected_after) and
  process_title::set_server_title from pub(crate) to pub.
- Move the mission-continuation prompt asset's accessor into base's prompt
  module (pub const MISSION_CONTINUATION_TEMPLATE) since the asset lives with
  the other prompt templates; mission.rs (app-core) now references it.

Build: cargo check/build -p jcode --bin jcode exit 0; binary runs.