feat(sequence): activation periods

Adds both of mermaid's notations: the activate/deactivate keywords and the
arrow suffix shorthand, where + activates the receiver and - deactivates
the sender. Activation counts are tracked per participant, so stacked
activations balance and deactivating an inactive participant is an error,
matching mermaid.

mermaid draws an activation box beside the lifeline. Columns are scarce in
ASCII and stealing them would push every arrow around, so an active period
is drawn as a heavy lifeline instead, with heavy-vertical junctions where
messages attach or cross. An activation left open at the end of the diagram
runs to the bottom, as mermaid's box does.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: cgreeno

cmd/testdata/sequence-ascii/activation_keywords.txt

@@ -0,0 +1,22 @@
+sequenceDiagram
+    A->>B: start
+    activate B
+    B->>B: work
+    deactivate B
+    B-->>A: done
+---
++---+     +---+
+| A |     | B |
++-+-+     +-+-+
+  |         |
+  | start   |
+  +-------->|
+  |         #
+  |         # work
+  |         #--+
+  |         #  |
+  |         #<-+
+  |         |
+  | done    |
+  |<........+
+  |         |

cmd/testdata/sequence-ascii/activation_shorthand.txt

@@ -0,0 +1,14 @@
+sequenceDiagram
+    Alice-->>+Bob: Hello Bob, how are you?
+    Bob-->>- Alice: I am fine and you?
+---
++-------+     +-----+
+| Alice |     | Bob |
++---+---+     +--+--+
+    |            |
+    | Hello Bob, how are you?
+    +...........>|
+    |            #
+    | I am fine and you?
+    |<...........#
+    |            |

cmd/testdata/sequence-ascii/activation_stacked.txt

@@ -0,0 +1,22 @@
+sequenceDiagram
+    Alice->>+Bob: request
+    Bob->>+Carol: sub-request
+    Carol-->>-Bob: sub-reply
+    Bob-->>-Alice: reply
+---
++-------+     +-----+     +-------+
+| Alice |     | Bob |     | Carol |
++---+---+     +--+--+     +---+---+
+    |            |            |
+    | request    |            |
+    +----------->|            |
+    |            #            |
+    |            # sub-request|
+    |            #----------->|
+    |            #            #
+    |            # sub-reply  #
+    |            #<...........#
+    |            #            |
+    | reply      #            |
+    |<...........#            |
+    |            |            |

cmd/testdata/sequence/activation_keywords.txt

@@ -0,0 +1,22 @@
+sequenceDiagram
+    A->>B: start
+    activate B
+    B->>B: work
+    deactivate B
+    B-->>A: done
+---
+┌───┐     ┌───┐
+│ A │     │ B │
+└─┬─┘     └─┬─┘
+  │         │
+  │ start   │
+  ├────────►│
+  │         ┃
+  │         ┃ work
+  │         ┠──┐
+  │         ┃  │
+  │         ┃◄─┘
+  │         │
+  │ done    │
+  │◄┈┈┈┈┈┈┈┈┤
+  │         │

cmd/testdata/sequence/activation_shorthand.txt

@@ -0,0 +1,14 @@
+sequenceDiagram
+    Alice-->>+Bob: Hello Bob, how are you?
+    Bob-->>- Alice: I am fine and you?
+---
+┌───────┐     ┌─────┐
+│ Alice │     │ Bob │
+└───┬───┘     └──┬──┘
+    │            │
+    │ Hello Bob, how are you?
+    ├┈┈┈┈┈┈┈┈┈┈┈►│
+    │            ┃
+    │ I am fine and you?
+    │◄┈┈┈┈┈┈┈┈┈┈┈┨
+    │            │

cmd/testdata/sequence/activation_stacked.txt

@@ -0,0 +1,22 @@
+sequenceDiagram
+    Alice->>+Bob: request
+    Bob->>+Carol: sub-request
+    Carol-->>-Bob: sub-reply
+    Bob-->>-Alice: reply
+---
+┌───────┐     ┌─────┐     ┌───────┐
+│ Alice │     │ Bob │     │ Carol │
+└───┬───┘     └──┬──┘     └───┬───┘
+    │            │            │
+    │ request    │            │
+    ├───────────►│            │
+    │            ┃            │
+    │            ┃ sub-request│
+    │            ┠───────────►│
+    │            ┃            ┃
+    │            ┃ sub-reply  ┃
+    │            ┃◄┈┈┈┈┈┈┈┈┈┈┈┨
+    │            ┃            │
+    │ reply      ┃            │
+    │◄┈┈┈┈┈┈┈┈┈┈┈┨            │
+    │            │            │