add session save and session load functionality

New [key-bindings]:
- session-save: captures cwd and foreground process argv to ~/.local/share/foot/state/{name}.json
- session-save-secure: prompts for a password, encrypts the scrollback with argon2id + XChaCha20-Poly1305 (libsodium) and writes it to {name}.scrollback.enc(stores up to 1Mb scrollback buffer).
- session-load: a minimal fuzzy picker that displays saved sessions (both secure and vanilla), UI piggybacks on search bar subsurface. use arrows to navigate and delete to delete a previously saved session.

session.h

@@ -0,0 +1,92 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stddef.h>
+
+/*
+ * Persistent terminal session state.
+ *
+ * Sessions are stored as JSON files under $XDG_DATA_HOME/foot/state/<name>.json
+ * (defaulting to ~/.local/share/foot/state). Each file records the cwd of the
+ * foreground process on the pty plus its argv, so a session can be resurrected
+ * in a new tab.
+ */
+
+struct session_state {
+    char *cwd;
+    int argc;
+    char **argv;  /* NULL-terminated for convenience; argc does not count the terminator */
+};
+
+void session_state_free(struct session_state *st);
+
+/*
+ * Inspect the pty for its foreground process. On Linux, walks /proc to grab
+ * cmdline + cwd. Falls back to fallback_cwd and a NULL argv on other platforms
+ * or when introspection fails (the caller will then save cwd only and let the
+ * shell start fresh on resume). Returns true if at least cwd is populated.
+ */
+bool session_capture(int ptmx, const char *fallback_cwd,
+                     struct session_state *out);
+
+/* Persist to <state-dir>/<name>.json. Creates the directory if missing. */
+bool session_save(const char *name, const struct session_state *st);
+
+/* Read <state-dir>/<name>.json. */
+bool session_load(const char *name, struct session_state *out);
+
+/*
+ * List existing sessions. Returns a malloc'd array of malloc'd names
+ * (no .json suffix), sorted. Caller frees with session_free_names().
+ */
+char **session_list(size_t *out_count);
+void session_free_names(char **names, size_t count);
+
+/* Returns true if name is non-empty and contains only [A-Za-z0-9._-]. */
+bool session_name_is_valid(const char *name);
+
+/* Remove <state-dir>/<name>.json. Returns true on success. */
+bool session_delete(const char *name);
+
+/* True if <state-dir>/<name>.json exists. */
+bool session_exists(const char *name);
+
+struct terminal;
+
+/*
+ * Read <state-dir>/<name>.scrollback.txt into out_text and out_len. Returns
+ * false (with out_text set to NULL) if the file is missing or fails sanity
+ * checks. Caller frees out_text.
+ */
+bool session_load_scrollback(const char *name, char **out_text, size_t *out_len);
+
+/* Best-effort delete of the scrollback sidecar (used when removing a session). */
+void session_delete_scrollback(const char *name);
+
+/* Encrypted-sidecar I/O. <state-dir>/<name>.scrollback.enc */
+bool session_write_enc_blob(const char *name,
+                            const unsigned char *blob, size_t len);
+bool session_read_enc_blob(const char *name,
+                           unsigned char **out, size_t *out_len);
+bool session_has_enc_scrollback(const char *name);
+void session_delete_enc_scrollback(const char *name);
+
+struct terminal;
+
+/*
+ * Called from search.c when the user presses Enter while the search bar is
+ * in a session prompt mode. Reads the typed name from term->search.buf and
+ * performs the requested save or load, then dismisses the prompt.
+ */
+void session_prompt_commit(struct terminal *term);
+void session_prompt_cancel(struct terminal *term);
+
+/* Finalize a save after the user has confirmed overwrite (y). */
+void session_prompt_confirm_overwrite(struct terminal *term);
+
+/* Picker (session-load) helpers. */
+void session_picker_init(struct terminal *term);
+void session_picker_free(struct terminal *term);
+void session_picker_refilter(struct terminal *term);
+void session_picker_move(struct terminal *term, int delta);  /* +1/-1 etc */
+bool session_picker_delete_selected(struct terminal *term);